Adviser or advisor?

Since waaaay back in the days of WordPerfect 5.0 (!), I've wanted to spell the word as "advisor", but the spell checker always highlighted it as a problem, giving me the response of "adviser". Okay, I would change it, but my fingers would always want to type it with an "o". I didn't have any problem with spelling other -er nouns (lawyer, philosopher, farmer, etc.), and I didn't have problems with spelling -or nouns either (doctor, professor, elector, etc.). So why did I want to spell advisor incorrectly?

Was it something that I had read as a child? Was it that I had always wrote it that way on paper, and my teachers didn't correct me? I mean, I don't pronounce -or endings differently than I pronounce -er endings, so it can't be an issue of mental pronunciation, so what was it?

Over the years, I realized that many other people spell it the same way as I was wont to: "advisor", and I knew that they would likely get the same result from MSWord the squiggly red misspelling underline, exactly like I was getting:

It was a problem that stuck with me until - on April 16, 2012 - I decided to go to my source of "spelling preference trend": google n-gram, and looked at the change in occurrence - in American English from my birth year of 1977 to 2000 - of "advisor" (blue line) vs "adviser" (red line):

Well, that explains it: although "adviser" is historically accepted version (peaking in the 1930s), "advisor" has been making continued upward progress since it came on the scene in the 1860s, and only recently surpassing the historical preference. In British English, the trend is a little different, through, with "adviser" still being highly dominant. However, the Brits also spell -or words with an -our, and many -er words as -re. ("Thanks", though, to American spelling hegemony, these are also changing, with even the British spelling of aluminium dipping to the American spelling.)

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Interestingly, MSWord has finally seemed to acquiesce in the 2007 and 2010 version: no longer is "advisor" given the squiggly red misspelling underline; both the -er and the -or spellings are now valid. Regardless of the continued dominance of "adviser" in British English, MSWord seem to have allowed both spellings to be correct in the English-United Kingdom setting...