There have been problems with post counts in the past, and it seems they still exist. Although this is probably the most notable example of a post count decrease (26 posts). You might want to pm JP and see if he can change it manually since it is such a large difference.

My posts do not count!

There were some links I posted in that thread that may be able to help you out:

Post number decrasing again
Post count
Deleting/editing messages

Good luck fixing your post count