In the last few weeks of my pregnancy my FH measurements went up an down by up to 5cm. It all does depend on where the baby is lying. Bum out meant I was bigger that week or if she was stretched out a bit more it meant I was under.
If you are getting loads of movements still then I wouldnt put much stock into those measurements.
I had my first three kids in NZ and they dont even do the FH anymore because it is so inaccurate.