martin wrote
[...]
It just happened that my wanderings yesterday took me on some buses -
the first two were charged at £1.20, then the third was 95p - meaning
that I'd been capped at the z2-6 off-peak Railcard cap of £3.35,
rather than the £3.90 adult bus & tram cap.
[...]

I think what is going on here is a system rule which is nowhere stated.

"No backwards causation" perhaps or "no subtraction"

ie, capping must work so you aren't charged more for a different order
of journeys and thus an additional journey can't reduce the cost.

In the current case if you journey on U'ground or O'ground z2-6 before
reaching the cap (which is the smallest railcard cap) then you must be
capped at £3.35. If "rail & tram only" means a higher cap then a
U'ground or O'ground journey /after/ going above £3.35 will require a
negative fare to be charged to get the same result !

And your observations causes me to wonder if things are different if
the bus journeys were before 09:30.

For a similar case, see a supermarket offer of "buy three and the
cheapest is free". The discount should be given at the very end to
avoid the result depending on the order in which items are scanned if
you buy more than three.

--
Mike D