I am 33+3, and am on fortnightly checks with the midwife due to being a bit of an old bat for a first timer.
Two weeks ago (Wednesday), I had a midwife check and measured at 90th percentile, thought this was a bit odd as I have been 50th percentile all the way through. Two days later I had a growth scan, and measured at 50th again, and the sonographer said "couldn't be more average if you tried" 
which reassured me no end.
Today I went for the fortnightly midwife check, and she has me measured at 100th percentile! When I queried it, and pointed out the scan size two weeks ago, she just said "Oh well, it could be me!" and laughed, but didn't alter my notes or re-measure.
I'm tempted to just ignore it, and put it down to the inaccuracy of the tape measure method, does that sound reasonable?