The records suggested that there was a wrong extension fare set at £4.15 between Zone 2 and Leighton Buzzard, from the following observations:
The return journey was charged at £4.15
The outbound journey was capped
When £4.15 was taken off the outbound journey, the result was a Zone 1-2 cap at £8.90
So the system charged a Zone 1-2 cap and the extension fare both way.