This answer is supposition, unconfirmed, and probably better as an advisory comment.
Nevertheless, I will offer it properly as an answer.
East and West can easily be defined with reference to the apparent motion of the sun across the sky: it rises at one horizon and sets at the other.
North and South are rather more arbitrary. You can use climate or geology, but those are rather small or limited to local observation. You can use magnets — but that distinction is symmetrical. Even if you had a sacred artifact which was clearly labelled THIS SIDE SHALL BE NORTH, you could have polarity reversals if the electrons were compelled to re-orient their spins.
Fact is that distinguishing electromagnetic polarity is one of the more obviously relational properties of matter.
Indeed, due to events not well understood by contemporary scientists — i.e. they aren't even quite certain as to how the Earth's magnetic field is generated, — the Earth's magnetic field has apparently undergone something known as a “geomagnetic reversal”, where the magnetic north and south drastically and rather abruptly swap places. I.e. the polarity is reversed.
There are ways to measure such reversals, and they don't contradict what I said earlier about magnetic polarity being symmetrical and nomenclature arbitrary.
It is quite possible that Mr Stephen King knew about such a thing, and that he deliberately made the compass in the Gunslinger's world a chiral mirror of ours as a way of distinguishing it from ours. The Gunslinger's world, being older, has probably experienced more such geomagnetic reversals than ours, and it quite possibly is in the middle of a period where, if it were placed alongside our own, would have magnetic north where our world has magnetic south.
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