The 360 version has around 25 100% reviews, the PC version has 10. Strip those out and I'd imagine the scores would be pretty equal. A perfect score is meaningless anyway- you're essentially saying 'this is the best game of all time, and nothing else can ever beat it'. In addition, a number of those will be 5/5 star reviews and the like, which Metacritic blindly converts to 100%, which is a poor way of aggregating review scores.