This is me hypothesizing as I obviously don't have access to the internals of Google. However, I do work in a related field to search engines so this is an educated guess.
Google's web crawlers indexed the page at some point when that question was on the front page. That question is the rare case where "Stack Exchange" was included in the title itself. Google is most likely looking at all the data that the web crawler pulled down (namely, all of the questions on the page when it was crawled) and sees the phrase "Stack Exchange" and assumes that's what we're googling for.
You can see from the spot that I've underlined that they indexed the page and found this question on 10/17. There's probably some calculus going on in the background that we're not privy to, but they probably think that because the phrase "Stack Exchange" shows up that this is the most relevant part of the page. As the index of this page gets older it will eventually go away, as @DavePHD pointed out this question does not show up because it's a 3+ year old question.