Could this just all be a massive test?
--Google throws mobile users onto sites where it expects them to be a good match, and takes notes on the results.
--As a control, Google occasionally throws mobile users at sites where they don't expect good results, and takes notes about that too.
The big problem I have with this theory is that surely it's been going on too long for that. It seems like as long as some people have been seeing this on their sites, Google would have learned everything they needed to know.