Similar to Dave Sutcliffe, my ratings for any individual game are influenced by other games I've played in the jam.
I judge the game as it is, not what it could become with more work. Not everyone had the same amount of time to be able to devout to the jam and not everyone has the same skills or the same size of team, I don't want to take those things into account. When a game is published the player usually doesn't know any of those details and they also don't directly affect the experience of playing the game.
At the end of the jam, the submission needs to stand on its own. If I think something could become a great game with more time and polish I may say that in the comments. But I won't score it on what might have been or could be, just what's there at the time I play it.