Arthur Figgis wrote:

An obvious difference is that Shepperton is SWT, while the others are
Southern - maybe Southern played decided to nice(r)?

Epsom is run by Southern, with SWT very much a tolerated visitor (in the mid
1990s it had the absurd situation that the SWT timetable was not available
at Epsom!).

Tattenham Corner is just about walking distance to Epsom Downs, could
there be risk of skewing journey patterns if one was Oyster and one
wasn't?

It's quite a distance and a good chunk of the direct walk is over grass and
squeezing round bushes and golf courses - it's hardly a pedestrian friendly
route.