If you press Achievements in the main menu, it brings up the Achievements window.

Underneath Achievements, mine says 600 Favor points.

Under that, I see a list of achievements, and a number to the right of each box.  Am I incorrect in guessing that that's favor you're supposed to gain when you gain the achievement?

In any case, I started a 2 player multiplayer custom game with a friend, against AI.  At the beginning of the game, I had 600 favor.  I then got the swift anklet which I had already previously bought (so it shouldn't cost any favor, right?). Part way through the game, I got the Corpus Explodicus achievement (200).  When the game ended, the screen said I earned 100 favor.  However, when we started our next game, I still had 600 favor.  What gives?