there were a LOT of things about the old forum software that either didn't work "right" or just didn't work at all.
we noticed some major differences when we put up the test site in terms of post counts being off, and deleted PMs all of a sudden showing up.
at this point, i have no concrete answer as to why these things occurred, but we're looking into it.
however, it will probably turn out to be just be water under the bridge and there is no fix for post counts (but there is for PMs - just prune 'em out).
that's about the best i can do for now, sorry.