vet error, surely?

My little Gizmo had be admitted in to the animal hospital 2 weeks ago due to potential lily poisoning (although we were sure he hadn't actually eaten any, better to be safe then sorry). 

He's fine, kidney's came back completely fine, and 2 days ago we had to take him in for his 1 week bloods. Whilst there he was weighed, which is where my question comes in. 

The vets are telling me that my little boy has lost 400g weight since we took him home, and they are worried about him. Admittedly we don't weigh him at home, however we have noticed no change in how he looks, he still eats the same amount, and he feels heavier more than anything else. 

My question is, could the vets have just noted his weight down wrong? Cause he really seems no different to me.

I've attached a picture before the vet, in the vet and now just for reference. 

Yes, absolutely, there are things that could have thrown his weight off. Do you know if they gave him fluids before weighing him? My last (check-up) visit to the vet with Lucky, he was showing as weighing over 9.5 lbs (indicated over a lb of weight gain in less than a week). He'd barely been eating so there was no way that was true. Then I remembered i gave him fluids earlier in the morning that threw the numbers way off.

If not, then... mmm, that's 14 oz, close to a pound. Do you know how much he weighed to begin with, and how old is he? If he only weighed 5 lbs to begin with, yes, that's very concerning. If it's 15 lbs, then an eye should definitely be kept on his eating but it wouldn't be nearly as concerning.

How long was he at the hospital, was it overnight? He may have been a bit stressed out possibly which can make them eat less. But if he's eating normally, he should be ok. They may just want to check him again though to make sure.

400g in two weeks sounds like a lot to me. But he's been eating normally and looks healthy, I don't think I'd be too worried right now.
Perhaps it would be a good idea to weigh him at home from time to time so you have a baseline for future reference. We do that a few times a year with our cats. It's not as precise as the vets, but it's good to know.

If it give you some perspective, my cat lost 200g in a bit less than a week by pretty much completely swearing off food....