Perhaps because Tattenham Corner is the end of that particular line?

I'm sure that's it - all the stations mentioned are termini of lines
that are partly in GL (and there are no competing operators) save Upper
Warlingham (which would be daft to be outside when the Whyteleaves are
inside). So it's a bit of an Epping or Hampton Court effect. On my maps
Watford is farther out than any of these.

That leaves Shepperton as the only such terminus not covered in this
way AFAICS. Make those TV audience wannabees urged to head there pay
extra?

In that area, I don't understand why stations from Kingston to Fulwell
are in Zone 5 (as they geographically should be) rather than Zone 6,
unless they're trying to keep all the Hamptons together (no pun
intended.)

Then there's Dartford. It's hard for customers on the 3 north Kent
parallels to get from one line to the other without going back to
Lewisham which might not be in their zone. Is it permitted to change
trains at either Dartford or Lewisham on a travelcard if you promise
not to leave the station?