I don't understand how my game's ranking on a "Top ___" list goes up an down. For example, I hit 300 plays in one day, maybe one of my highest yet and I'm placing lower than average on the "Top HTML5 NSFW Games" list. Is there any documentation on how the algorithm ranks the games?

No. The algorithm is designed to be unpredictable on purpose. That's for two reasons:

1. So people don't try to game it.
2. So that more games get a chance to shine, at least for a while.

Focus on making your game awesome, and market it well. That's how you take a shot at success. ;)