There is a nasty bug in Giana Sisters that allows you to grind achievements again and again and again and your Gamer Score rises... I would report this to the devs but I have not access to their forum yet.
What happens when I start the game it instantly disconnects me from LIVE and somehow I can't log back in if the game is running. I can play the game just fine and it gives me all the achievements, BUT when I quit the game and reboot the xbox all the achievements I've got from the game are gone. Gamerscore holds them. So when I start the game I can get all the achievements again and Gamer Score rises again.
This should be fixed asap.