It's literally a typo in most cases, or an error of ignorance, or a spelling error codified into accidental tradition out of habit. It's not any different from the authorial choice to use Imperial instead of metric, except that we went after that one much earlier in our site's evolution.
And it makes stuff like this happen: http://05command.wikidot.com/forum/t-5787987/non-disc-record-nightsmaiden
So it's a classic spelling error that adds nothing, and occasionally causes newbies to get scolded for no good reason, while taking up precious staff time dealing with reversions (staff time used for reinstating spelling errors, no less). Not an emergency situation, but I come down on the side of fixing the problem.
I support changing the error, like we would any typo, and letting authors revert it if they wish.
If we DO keep it, we can't just leave the explanation in the FAQ if we're going to make staff take the time to enforce the spelling error. We should put it somewhere like How To Write An SCP (like not_a_seagull suggests).
Additionally, if we do keep it, I think staff need to stop advising newbies to change the spelling, at least as long as they're carrying out their official roles.
That said: This is a really trivial issue. While I think there's a clear right answer, it's not the end of the world if we keep ignoring it.