This is because as you know, we n Undertale, if on a pacifist (or maybe neutral idk if he appears in neutral either) route when sans appears before you enter the MTT Hotel to take you to a shortcut, and the music like somber-like plays and the title is called “It’s Raining Somewhere Else”, and after you defeat the titan in Deltarune Ch4, the somber music that plays is titled “The place where it rained?” (Ooooh, chills). First and most obvious point (that everyone has known for like years) is that sans originally came from Deltarune. The second point is that Undertale and Deltarune universe timelines are closely fit, as when Frisk goes on the (date?) with sans, Kris goes home to sans getting drunk with their mom.
I’m probably just pointing out the obvious at this point but like it’s a good idea. So, by that logic, when Frisk fell into the underground, Kris was falling into the castle town dark world. However, my theory isn’t that they will start and end at the same time, it’s that they are relative to each other, starting at the same time.
However, this theory is literally just because of those to song titles, so don’t expect it to be that credible/sensible/good/interesting.
