Oh right, so it really is a calculation of points vs age, according to the formula, plus penalties -- which my analysis didn't catch. Thanks for the background.
Hah, good point! Much to my chagrin, the theory I proposed is challenged by my own post. It looks like someone with a lot of karma can boost a post to the front page (there are some outliers in the analysis). Maybe we can get individual votes exposed via the API to know for sure.