Per the game log, there were 5 FGs in your deck, 2 in the other guy's, and 3 in the trash, adding up to 10, no?
The CouncilRoom stats are just confused, probably -- maybe you should follow up with rrenaud.
IIRC, Thief doesn't take anything from the supply; the treasure you "gain" is the one the other guy "trashed" (admittedly, not the best wording...).
Maybe the Possession screwed up the FG count, though I'm not sure why that would happen and it's really just a random guess.