Your Word and Excel formulae can both be reduced to:
=1-((1+2/100)*0.995)^19
and, with the appropriate formatting switch applied to the field in Word:
{=1-((1+2/100)*0.995)^19 \# 0.00000}
will both return -0.32447. That said, there does seem to be a bug in the Word calculation when you unnecessarily use (0.995) in the calculation; that seems to cause Word to treat the 0.995 as a -ve value.

I don't see what this has to do with a mailmerge, however, as no mergefields are involved.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]