Need info about chronic eye discharge

I am having trouble understanding something. From what I'm reading it appears as though a cat with eye discharge can be treated with either eye drops/ointment or with oral antibiotics. However, some cats with eye discharge will only benefit from treatment with oral antibiotics. Why is that? Why is it that in one case eye drops/ointment will cure the eye discharge, but in another case only oral antibiotics will work? Does this having something to do with the type of infection the cat has? Please explain.

I'll give you an example of what I'm talking about. One of my cat's had a chronic eye discharge for three years (from birth) despite being seen and treated by two veterinarians. They said it was a viral infection and suggested using L-Lysine. One of them also prescribed an antibiotic eye drop but it stopped being effective after I was instructed to stop giving it to her. A couple years later a third veterinarian prescribed an oral antibiotic which seems to have cured the eye discharge. So basically this cat suffered with this chronic eye discharge for three years before finally being cured with oral antibiotics. I'm wondering what that means. It sounds like she had some type of bacterial infection that went undiagnosed for three years. That doesn't sound good to me and I can't help but wonder what that has done to her overall health. I also can't help but wonder if this has affected her eyesight in any way.

I adopted a cat from a co-worker that she had had for about 5 years. She said he always had an icky eye. So when we got him, I took him to the vet, assuming he had herpes in that eye. But the vet said, no, it was just a blocked tear duct, and try and clean that spot (nasty crusties all down the side of his nose.) Naturally, I could never really clean it.

In November of this year, (It's been 2 or 3 years we've had him), he got an abscess (on his side) and had to be taken to the vet. He had oral antibiotics, and cream for the wound. Well, wouldn't you know it...his eye problem cleared up. It still "overflows" like he's crying...but it doesn't get black and crusty. I really don't know why, but otherwise he's totally unaffected.