Yeah, it doesn't sound right to me either. I suppose there's just no way to determine how big a horse will get so all I can do is wait until next year and see how much she grows. I do think it will be more than an inch or two.
The vet's prediction sounds weird to me. My Cody was 14.3 at 11 mos. and he's now about 18 hds!! I did the knee to coronet band measurement, came out at 18 in. so it worked pretty well for him. I've also heard you take a string and go from the pastern joint up to the elbow, then take the pastern end and go up (keeping the elbow point in place) past the withers. That is supposed to predict their grown height. I forgot to measure how high in hands that would make him but it was about 8" if I remember right.
Well, he just turned 4 in June so I don't know that he's done yet but he seems to have slowed down significantly. He grew a lot up until around 3 1/2, then started slowing down, thank goodness!! I've heard sometimes they experience another grow spurt around 5, we'll see. I hope he's done, I can barely mount on the trail anymore without something to stand on!