Looks like numerical overflow. In hex the numbers are 0xfffffd, suspiciously close to the round number 0x1000000. Probably, the engine for some reason subtracted slightly more posts than added. (BTW, I had similar problem at wincmd.ru forum: when a post was copied using a special mod, the user's posts counter was not increased, making it off-sync with the real number of posts; so just copying then deleting a post led to decreasing the counter.)
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