Fundal height/measurements, how are they supposed to work?

Ok I had my MW appointment last week at 31weeks & I'm not sure I understand how the fundal measurements work exactly.

I initially measured 35 weeks 3 days. So the midwife poked & prodded bub & did another 2 measurements & took the lowest at 32 weeks 5 days.

I know fundal measurements are reasonably subjective but should I be worried that I was a month ahead initially? And is this normal for them to move bub around before taking the measurement?

Well, for me, DD never moved. I also had low fluid. I was 33 weeks and measured at 24.

And bub was 30 weeks in size,

I wouldn't be too worried.

My measurements were always under. And I had a relatively large baby (for my frame I mean, she wasn't massive - 4kg).

I think they're a load of old rubbish but am sure there is a reason they still do them :)

ive always measured 3 to 4 weeks under and last check bubs was transvere so she really couldnt measure me
and i have big babies
mainly its just a rough check
i think its more if your measurements dont change or change dramatically which then they will do an U/S