Uezyteue

The reason I say "-ish" is because it also pertains to circumstances in Undertale.

The thesis here is simple: Sans and Papyrus (or at least Sans) are/were dimension-hoppers, and not strictly from Deltarune.

To begin, I'll start with the biggest point of contention: where did they come from? In short: I don't know. I have no clue where they might've actually originated, but I think they're not from Deltarune or Undertale originally. I think that, whatever that machine is in Sans' basement, it's been in use for a lot longer than people think.

My evidence? Mainly, it's the way that Sans refers to Papyrus in Chapter 5. When you ask him about his brother he says, "Yeah, my brother. He'd love the festival... shame he couldn't make it this year." After Susie gives him the punch card, he says it's not worth anything, but now he has a "souvenir" to give to Papyrus.

Now, if Papyrus was just reclusive, that'd be fine and good, but the progression of interactions with the door seem to indicate that we're gradually getting farther from seeing Papyrus. Additionally, Papyrus is notably not reclusive whatsoever. If he knew there was a festival happening, I don't think there's a force in the world that could keep him indoors. Finally: if Papyrus is just inside the house, why is the "souvenir" punch card in the basement? As far as anything seems to indicate... Papyrus isn't in Hometown at all, and Sans has somewhere to go back to.

As far as I can tell, at least by the time of each game, Sans and Papyrus were in Undertale for a lot longer than they've been in Deltarune. In Undertale, they just showed up "one day" and asserted themselves. Evidently, this was long enough ago for them to establish a general rapport among Snowdin and Waterfall, and also go through Flowey's "experimentation." Meanwhile, in Deltarune, Sans is very clearly new in town. The house is "new," according to Susie when interacting with the door during the weird route intro, and apparently "someone else" was manning the Ferris wheel just last year. Furthermore, and this one's pretty obvious: Sans' grocery store is just Grillby's but remodeled, he just scratched out the name and added his own on the end.

Furthermore: in the genocide route of Undertale, Sans proves to be very knowledgeable on the topic of timelines, and knows that they can be crossed and manipulated. He's also very adamant that just doing such massive things for the hell of it because you can has big consequences, almost like he's been through it before. He's faced the consequences of time manipulation going wrong, and he knows that you will too.

Papyrus, seemingly, is also in the know about these things. Apparently, Sans has used the ability to manipulate time and space to play pranks on Papyrus in the past, and he knows that this is what happened.