Hi Feathers,
It’s the same across the whole of zones 2-6 on late adopting NR routes. Any 2 zone fare.
The reason is that at every fares increase the PAYG single fares are increased by the standard amount with rounding to ensure a multiple of 10p. The paper return fares are also increased by the standard amount with rounding, but as the fares in question are twice as much the rounding is often less of an issue.
I’ll see if I can get in touch with someone who can flag it to appropriate people.