Fundal Height question!

So, I had my 34 week appointment today. Baby had a fundal height of 29. Doc scheduled an ultrasound to do a growth scan at my 36 week appointment.

But, the thing is, at my 32 week appointment, fundal height was okay at 31 cm.

Why is it two weeks less? I know it could just be the way she measured this time, but it kinda worried me that it was less than last time.