There can be a lot of difference between your scales and the vets'...

How about trying this? Get an item that you KNOW how much it weighs on your scale...like a bag of sugar. Weigh it on your scale, then try and pop by your vet's place(the bag of sugar won't change its diet or gain or lose in the meantime! )

Then you'll know if there is any difference!

Just an idea.

Catty1