That's a possibility, yes, but only one person reviewed my game before the voting started, and such person said that they actually rated the game afterward. However, I don't doubt that forgetting to rate a game is something that happens often.
I have joined some jams too, but for me, this time is more noticeable than usually, that’s why I pointed it out. it's just more fun when more people engage, you know, and you get more exposure and feedback.