Was the cat diagnosed with an upper respiratory infection or urinary tract infection?
Seems there was some confusion as this thread continued, but I'm assuming since the OP wrote URI, it was the former.
Also, it wasn't mentioned if it was tablets or the liquid suspension that was prescribed.
URIs in cats can be very difficult to diagnose. UTIs are easier to by taking urine samples.
If this was a new vet for your cats, who knows why he/she prescribed what they did. If asking them didn't get you an answer, keep asking, or find another vet. It's been my experience that some vets are not good listeners and like to act superior to their clients. I've not returned to several vets for just this reason, and have complained to one 24 hour emergency practice about the rude behavior of first vet we saw there. That condescending vet assumed I didn't know what I was talking about when I told her what I had witnessed, because, by the time I got my cat into the emergency clinic, the seizure was over. The vet looked her over and then sent me home, saying it was likely just her leg that had fallen asleep (huh, NO!). The next time my cat seized was 10 days later, again late at night. We went back to the emergency clinic and this time I saw the owner vet of that clinic (at twice the cost of my regular vet's exam charge, btw). Again, the seizure was over by the time we got there. Anyway, I told him of the previous experience with the other vet, and he looked dumb founded by her diagnosis. He said it was highly likely that what I had seen both times was a full on epileptic seizure. I thought that this was odd as my cat is 13 years old and I'd never seen her seize before. However, he explained that just because I hadn't seen the seizures doesn't mean she hasn't been having them. Likely, she'd been having what's called petite mal seizures and and now she's having grand mal seizures. Epilepsy can get worse with age and no one really knows what causes it. After doing a lot of research on the subject, I have learned that all my cat's life she was having those petite mal seizures, and I just thought she was a high strung cat. Anyway, the male vet also sent me home after about an hour of checking my cat over and running some test, but he kept my cat for several hours more, just to observe her. When no more seizures happened, at least in his presence, he called me to come get her and said to try and film her the next time I saw a seizure coming on. He also told me what to look for in the aura stage, the pre-seizure behavior. Well, the next time I saw her have one was 2 weeks later, during a week day, luckily. I did notice the aura stage coming on, but by the time I found my camera, the seizure was over. The next time was about a week later, but I couldn't get the stupid camera to go into movie mode. So, I called my regular vet to tell them what was happening and how I'd been to the emergency clinic twice. Funny thing was, the emergency clinic never faxed over any record of my visits there to my regular vets' offices, despite them saying they would do so the following mornings. Anyway, luckily for me and my cat, the vet I spoke to happened to also be an epileptic man and I finally got her on some medication. Since then, no more seizures, and my cat is way more normal now than at any time in her previous 13 years of life. She's calmer, sweeter, and not a freaky girl anymore.
My point is, there is no reason why any vet shouldn't tell you why he/she prescribed one thing over another. As for taking your cats to another vet for a 2nd or 3rd opinion, that's always your right to do so.
So, assuming it was a URI, the reason for the Orbax Rx may be because it has been shown to be very effective for a number of bacterial infections, not just skin infections. It's very easy to administer in the oral suspension as it's tastier to cats than Clavamox, doesn't need to be refrigerated like Clavamox suspension, and doesn't go bad as quickly. However, if giving any tablets or liquid hasn't been a problem for you or your cats in the past, then bring that up with the vet, and also let them know that cost is a factor too.
Generally, tablets are cheaper than liquid suspensions, but many owners find it difficult to pill their cats; so having liquid options are great. Also, some meds in tablets are too difficult to split into the dosage needed for smaller animals. So, it's great when they come in liquid forms to begin with. Yes, you can get the pills compounded into a liquid by a compounding pharmacy too, but that will add cost, though it might be cheaper than the original liquid form from the vet's office. I've had some issues with both my cats being allergic to some meds after they've been compounded, where as in tablet form, they were fine.
Also, if your cat can wait a day or so before going a medication, price it online. They are always cheaper than what my local vets charge.
Why the vet you saw prescribed Orbax over Clavamox could be for a variety of reasons. Resistance to antibiotics is one reason. Another could be cost, because yes, vets upcharge for all medications, sometimes by 2 or 3 times the price I find it online. Orbax is a newer medication and therefore pricer anyway.
Do your cats have any underlying long-term disease, such as FHV (feline herpes virus), or anything else?
Reason I ask is FHV is very, very common these days, and secondary bacterial infections are common to cats with FHV, and hard to get rid of because the virus eats away at the mucosa membranes, as can bacteria. Also, some strains of bacteria have become drug resistant, so they are more difficult to treat. Also, there is still no cure for feline herpes, or any herpes to my knowledge.
If all your cats were getting sick at the same time, then if one cat has FHV, it may have passed it to the other cats. However, there are other viral and bacteria infections that can easily pass from feline to feline too.
Often, if an infection doesn't resolve with one course of meds, there are 4 things that can be done:
1. Stay on the same medication for longer;
2. Up the dosage frequency;
3. Up the dosage strength;
4. Change medications.
Both my cats have FHV, and as such, tend to to have some FHV symptoms at times - stuffy noses, sneezing, weepy eyes, wheezing, and coughing. My cats are currently on Orbax for two reasons. Angie, nicknamed Sneezy-Wheezers (due to the FHV) has a UTI that didn't resolve after 10 days on the Orbax; so I'm doing another round of it for her. She did initially improve, but it came back a 2 weeks later, probably because she wasn't on the medication long enough. Her FHV induced wheezing has concurrently subsided too, as an added bonus.
The other cat, Tibby, is battling a secondary bacterial infection in her sinuses, which she often has, also due to the FHV clogging up her sinuses. Tibby is also the epileptic cat. Her nickname is just Sneezy, but she also has the weepy eyes almost all the time. Tibby is prone to the sinus issues and only doing long term antibacterial therapy (a month at least) will resolve her problem, at least for a few months anyway. I've dealt with this for many, many years, so I'm well versed on the subject. Orbax works for Tibby's sinus problems better than any other medications we tried and it's very easy to give in the oral suspension.
And just so all you readers know, my cats are not on any dry kibble. I only feed them very high quality, human grade, wet cat foods.