A lot of people play and don't rate. I got 13 browser plays and only 4 ratings.
Viewing post in Are most people actually playing the games?
You sure the 13 people were participants? 4 out of 13 is a big difference. In my case, I don't have a browser version, only windows version to download, like 6 people downloaded my game, 4 reviewed it, and 2 rated. I think I have no way of knowing if those 6 were participants, but the reviews were participants fore sure.