Hi,
Possible reason is,your result can contain a null value, and if that null value is in the 1st row, then the field/formula is defaulting to a string when exported.
IF ({AEMPPF02.IDPRT#} = 'FRTCMB') then
ToNubmer({AEMPPF02.EXTSHP})
and check below options in File > Report Options
Convert database Null Values to Default and
Convert Other Null Values to Default
--Praveen G