It's perfectly normal. My friend had identical twin girls and one was always bigger than the other. I wouldn't worry yourself too much about it since it's a common thing.
If the measurements are off with women who have 1 baby I could only imagine how off it they are with women carrying two babies. Babies sometimes have growth spurts and no two babies are completely the same, even twins cause then you wouldn't be able to tell them apart. I say don't worry about it for now.