So the study tracks...
...people who leave WiFi on while on the tube and not all the commuters. How representative is this sample? How clueful in general are those people? How attentive are they to the signage/patterns/whatever might help them shave a couple of minutes from their daily commute? Would disabling WiFi be more beneficial than investing in better signage?
NB: tracking Oyster cards means tracking regular commuters, not occasional visitors, tourists, and such, so that is also biased.
Actually, I wonder if the "WiFi-on" populations has a disproportionally large fraction of foreigners who do not have mobile data plans and are happy for any free WiFi between sights? [Not a Londoner and my WiFi is usually off, so I don't even know if you can register on the TfL WiFi with a foreign mobile number.]