Short answer: People tend to be nice in jams so you vote for their game.
Long answer: Besides that, if you're "too honest" with someone they might get mad and give you negative votes just because they put too much effort and got offended (it happened to me, LOL).
Also the voting system is... odd (borderline bad and easy to rig), if you and your friends just open accounts and vote for your game (and upload a tech demo so they don't get disqualified) you win. I've seen INSANELY looking games, super creative ideas, original mechanics and yet... the #1 is a die/respawn basic unoriginal platformer.
My advice? don't look at the ratings of big jams, a lot of people cheat with alt accounts, or vote you 1 star just to lower your rating. Small jams have more consistent ratings and have more awesome communities because they're not as competitive and people genuinely help each other and give more honest and constructive feedback.