I'd guess it would only logically occur in the instance of a worldgen having both poles. If it lacks a pole, then we could simply assume that the "world" is only the extent of the known world and not the entirety of the globe.
Any anomalies between the world size and the implied size of the globe (based on the presence of both poles) could either be abstracted away, or you could think of whatever desired excuse there is for the fact that a pocket-sized world could contain both poles while still having Earth-like gravity.
Then again, since the world in DF is always quite literally hollow (Hell and its delicious bottomless pits), this combined with the density of slade would allow for plenty of excuses to fudge the math in favor of ignoring any "how is this tiny planet still Earth-like" questions. :V
That explanation does make a lot of sense. AFAIK a lot of succession forts that are supposed to be in the same universe lore-wise (but use different versions) use the explanation that its just one part of the world - I've taken to that explanation as well when doing regens so I don't feel bad about leaving projects behind.
As for the bottomless pits in Hell, I'm guessing they aren't literally "bottomless". Either they're very deep, i.e Marianas Trench deep, so you just keep falling, seemingly endlessly, until you eventually come to an abrupt halt and become the accordion version of yourself, or maybe they're actually portals to wherever the clowns come from, hence why they glow (and why you are, for all intents and purposes, considered "dead' when you jump into one).
IIRC wasn't there some teasing about alternate dimensions in future versions? Maybe then the glowing pits won't kill you but instead be used to go into the true "Hell" dimension.