Hi Rob,
Although the off-peak cap for zones 1-9 is the same as for 1-6, the weekly caps are very different. I’d therefore expect you to be charged zone 7-9 extension fares relevant to the times of the journeys to/from Brentwood. However, they won’t account for £11.30 extra, unless you went there and back twice.
I’d be grateful if you could post your journey history for the week so I can try and work out what’s happened.