I severely doubt that any sort of mentionable testing has been done regarding whether or not the two direct accuracy enhancements stack. I'd chalk that up to someone saying it 'that one time', which ended with the rest of the summoner community blindly accepting it over time.
Of course, I'm basing this on the fact that there's really been no formal avatar accuracy testing at all, making the likelihood of there having been any testing regarding such a trivial matter quite low.
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