Maybe when top of piston is plenty of carbon deposits, engine performs better due to high compression ratio .....
Read this from Toni Clark (Zenoah distributor) :
....Experience has shown the power steadily increases over the first thirty flights, and
even after this time, the power will increase from season to season, due to the build-up
of carbon deposits on the piston crown and on the combustion chamber walls. This
carbon build-up increases the compression ratio. Because the engine is designed for
an extraordinary long life span, even when mineral based two stroke oils are used,
the designers had chosen a relatively low compression ratio to allow for a considerable
amount of carbon build-up. So "don't worry" about carbon build-up and as long
as it is not in the piston ring groove, do not remove it, instead "be happy" about the
extra power! .....
From ZG20 user manual ...
Regards