You probably had some "Thanks" in threads (or individual posts) that were deleted in the past. The "Thanks" counters do not automatically get recalculated when posts are deleted. However, once a year or so we go through and clean things up and the "Thanks" counters get recalculated correctly.
As you might have guessed, we've started our "Spring Cleaning".
Some people might still see a small diminish of their "Thanks" as we continue to clean up old deleted posts over the next couple of weeks.
Thanks for understanding,
Alex