Most likely it's because the shutter speed was 1/125th and it was quite windy, (I tried to wait for things to calm before exposing each frame) so there may be some motion blur in some frames.
I chose FP4 because it is so flat when dry, so there shouldn't be any issue with scanner focus. I might scan one of the other Xtol frames and see if it looks any difference. I suppose it's possible that FX-55 might not have the same grain solvent ability that Xtol has, due to the lack of sulfite.