A question I have answered some half an hour ago (Proving that all numbers are rational) stays deep below the fold and doesn't pop up to the top of the question list. Is there any explanation to this? Thanks!
Actually, a new answer bumps the question.
However, it all depends a bit on the way users view the posts.
If you look at the posts tagged rational-numbers, this question is on the top if you choose the active tab - which show questions that recently had some activity. However, if you choose the newest tab, the questions are displayed there depending on the date and time when the question was posted. (Still, this question is visible there, since it is relatively new and this tag is not user very often.) Of course, the same general principle is true for the active and newest tab when viewing the questions - I just chose this tag as an example because of the specific post you linked to.
You can find some further details and link to more information in the tag-info for bumping.
I will add that the page https://math.stackexchange.com/ is a bit different. (For example, it does not have active or newest tab.) See also: Proposal: make the “interesting” tab the default on Math.SE homepage.