I would expect most experienced a gain vs. a loss. Especially since the Odds & Ends are included in that count.
It could have been from deleted threads you had participated in.
Perhaps from when the old "Off Topic" forum was removed? When
threads are deleted, the post counts aren't adjusted (at least in the
old forum they weren't). However this morning I ran a script to
update everyone's posts to reflect actual posts in the database.
Looking at my own numbers, looks like I experienced a loss of
300. I was a regular participant in the old "Off Topic"
forum. Ah, the memories...