Reasons for Fundal Height Measuring Small

I had my 37 week apt today and a doctor who isn’t my normal OB said my FH was between 35-36 cm (not sure why she couldn’t give me a more accurate measurement). She had me in for a quick ultrasound and my fluid levels were good at 12cm and baby is moving fine. Just curious what else could cause a fundal height to measure behind? I’ve been measuring perfectly up until this point. :/