Alot of it is in how they measure. 5w ago I was measured a week ahead by a midwife, and this week I was measured a week behind by an Obst. He informed me that I have a small baby, but really, noone can know until the little sucker pops out!
There is easily a cm discrepancy in how the individual takes the measurement, and depending on whether you are carrying high or low makes a difference too.
I also believe that past 36w it makes no difference what they measure..... I think ?