How do the ratings work for the online challenges?

For example, Oil rush is the clear winner. But, how do some of the challenges manage to achieve a negative rating?

I haven't been able to spot anything that will allow me to rate a challenge yet...

muchas gracias.

My suspiction is when someone downloads a challenge to play, it gets a -1 rating. When someone completes it and submits their score, it counts as a +2. So the rating isn't a guage on how much people might have enjoyed the challenge, just whether or not people are winning it.

Some of the challenges still have plus score even nobody is on top list...

Well, I dunno. Perhaps a challenge gets a +2 if they submit thier islands instead of their score as well?