Troopermaster makes a great point here, as does mason.
I think that the HDPE helmets probably had a smoother finish in ANH, than they did in ESB.
then you have the factor of age and paint, and the storage elements produce even more
bumps, and paint chipping.
and the fact that the helmets were re-painted at least 2 times up until ESB was filmed.
in some shots you can clearly see that ears are put on wrong, helmets assembled with
different shapes to the brow height, and different types of eye cutouts...
all sorts of subtle differences.
paul is right about the noticable bump pattern, and that's the result of the mould buck.