The falling-out isn't between the main site and Area 51, but rather between Google and Stack Exchange.
Anna Lear, an SE developer, tweeted about a Google API change two days ago. That's when the moderator tools for viewing traffic reports stopped working. (These are still broken as of this writing.)
Why is some part of this chain reporting 20M visits? Possibly this is part of reseting something -- we may be seeing every page load since the beginning of time, though that still feels off by a couple orders of magnitude. However, another, much-smaller site is seeing the exact same number of visits now, so we might be getting all hits for the entire network over some period of time. Throw SO into the mix and things get out of whack, after all.
Area 51 sometimes goes days without updating this stat. Area 51 is old and creaky and doesn't get a lot of love; my guess is that it isn't as resilient in the face of failure or flakiness. Area 51 may have just given up, while the main SE sites try valiantly to get updated data from Google and, well, don't necessarily get the best data. This may be one of those rare times when Area 51 is better off for being old and creaky and devoid of love.
Alternatively, maybe we should blame caching. It's traditional.