There's a poster at my local station which says that, from 2 Jan 05,
Norbury will be in zone 3 instead of zone 4, and Belmont and Cheam will
be transferred from zone 6 to zone 5. This will be good news to people
who commute from those stations to inner London, but not so good for
those who travel in the opposite direction.

Does anyone know what determines changes like this?

--
John Ray