I know that it's a terribly ridiculous explanation for a simple problem.
I mean, hell, why don't we just say that Google has 47 different indexes, one for every country that they serve. The reason the SERPs are different for each localized region is because they're all pulling from different indexes!
To assume that they have multiple indexes instead of one index is a flawed assumption without any other evidence to go on. (Supplemental aside, which has a known purpose and is clearly marked, acknowledged, and is not related to the sandbox).