It’s up to individual judges how they determine their score. The scores from the judges are normalized before they are averaged together.
For me, first I like to classify the game, as being either superb, good, fair, mediocre, poor, etc. I simply do this in my mind, and once I’ve decided that some game is superb, I can quickly say it deserves at least 90%, and then I compare it to other games that have 90% score and decide that it is better than this but not better than that, and come down to a score of maybe 93%. There are many factors I consider, as ra4king mentioned, graphics, ease of play, difficulty, technical factor, audio, gameplay of course, etc. However relying solely on these factors to judge a game is not good science either, because the game could have audio, nice graphics, technically impressive, but suck in fun and gameplay. So there are aesthetics and subjective factors as well.
But it isn’t accurate science. It is easier to do this for the top games, because they are fewer and easier to compare. But the lower you go in the rating, like 70-80%, it becomes much more difficult, because there are more games that fall into that bracket and comparing them is more difficult.
There are always ways to improve the judging process, but year after year the results seem to be alright. I did suggest in another thread to use buckets like “Superb”, “Great”, “Good”, “Fair”, “Mediocre” and “Poor”, decide on what games fall into what bucket, and then order the games. The judges would have to coordinate together and agree, and that is the biggest practical issue with it really.