Usually is that way or sometimes just a person (the one who open the jam).
Ludum dare allow only to participants to vote
All participants that submit a game are allowed to judge. Games are given 1-5 star ratings in each category, or N/A where not applicable. The categories include:
Innovation – The unexpected. Things in a unique combination, or something so different it’s notable.
Fun – How much you enjoyed playing a game. Did you look up at the clock, and found it was 5 hours later?
Theme – How well an entry suits the theme. Do they perhaps do something creative or unexpected with the theme?
Graphics – How good the game looks, or how effective the visual style is. Nice artwork, excellent generated or geometric graphics, charming programmer art, etc.
Audio – How good the game sounds, or how effective the sound design is. A catchy soundtrack, suitable sound effects given the look, voice overs, etc.
Humor – How amusing a game is. Humorous dialog, funny sounds, or is it so bad it’s good?
Mood – Storytelling, emotion, and the vibe you get while playing.
Overall – Your overall opinion of the game, in every aspect important to you.
There’s a complex system that try to
force push people to play other games. The more you play (and vote) the more you score.
Other like Kenney-Jam are voted by Kenney himself (and maybe some other people of the staff).