The threshold of 2 upvotes is a necessary condition to be considered for the front-page (all pages, not just rank 1-30), not a sufficient one. I have this information from dang himself.

Garbage on the new-page will only be viewed, but never upvoted, so it drops quickly because of the view- and age-penalties in the formula and makes room for other submissions. But of course, this needs real-world experiments.