This, I have noticed some years ago. It seems much like, if the number of returned results doesn't meet a given threshold, some kind of optimizer runs over night on these searches in order to provide a more extensive result set.

Super interesting discovery! I wonder if whatever algorithm Google is using has reached its scalability limit on today's Internet, and it takes some kind of an over-night batch job to do obscure searches usefully. Maybe all Google Search is doing is just a giant cache of slow search results.

This has been some years ago. Notably, I observed this in relation to search suggestions. You could enter a search and get zero results, but a day later or two, you'd get at least a suggested search term (regardless how accurate or meaningless this may have been). So I guessed, these were built up, at least partly, retroactively. With results now happily including these "sympathetically adjusted search terms" without presenting this as an explicit option, I'd guess, this may now apply automatically.