There are several games I thought I had rated but didn't. I think this is a usability problem with the ratings system. It's easy for a user (or at least for me) to select star ratings, then type a comment and submit it before hitting the 'Rate game' button. Submitting a comment clears the entered ratings. It might be an improvement for the 'Submit comment' button to also save the ratings (which can then be changed and resaved later if necessary).
Of course, it could just be me! But I'm willing to bet there are others whose ratings were entered but not saved.
P.S.: If you think you might have made this mistake, you can scroll through the entries page and look for games you meant to rate. If their rating got saved, they'll appear at the bottom with the text 'Rated' over them. Otherwise, rate again!