Why do old posts show up in the First Posts queue?

Question

First posts almost invariably show in the queue of the same name within 15 minutes or so, and sometimes nearly immediately. Occasionally, however, when looking at the First Posts review history or reviewing some myself, I’ll see a post from days or weeks previously.

For example, this review was done on September 23, but the post in question was made two weeks previously, on September 9.

Why are such old posts showing up in the First Posts queue?

Answer 1

That user has posted two questions and no answers on this site. The first question (call it post A) was not the one from the linked review (post B). Post A was deleted earlier today, leaving post B as the only post belonging to that user which remained undeleted. Thus post B became, in a sense, the user's "first post" and was put into the First Posts review queue.