Don't worry too much, fundal height isn't an accurate science by any means. There are many variables with it, for example you could've just eaten so had a full tummy, you could've had a full bladder or had wind/gas that can push everything up and lead to a bigger measurement. Even human error. Ive just had the opposite problem, had a low tummy measurement last week at 36wks (I measured 32) I had a scan booked for Monday anyway and I was super nervous. I went along and my baby is measuring perfect, in fact a week bigger than my dates. They're just being cautious and look at it this way - you get to see your baby again before birth, that's always good lol. Best wishes :)
Which measurements are over? Your fundal height (tummy)? If so they will just want to get a more accurate measurement of baby, make sure he/she isn't growing too rapidly and also to probably check amniotic fluid levels...One or both of which can give a larger than expected fundal height reading.