The origins of "AA制"
When I lived in Guangzhou I was told the expression came from Hong Kong and stood for "Algebraic Average".

That also doesn't sound like something a native English speaker would come up with, however I don't think that's a reason to discount it (or even the other suggestions) if it came from Hong Kong where non-native English speakers come up with all sorts of part-English phrases that sound awkward to us but still become popular, like "他真没有sense" to mean "he has no taste" being one I heard recently.

Then again I can't be sure if "Algebraic Average" is the true etymology either, just offering my two cents.