Hi Alan (confused of Berkshire),
That’s a great question which highlights some odd features that aren’t often seen these days, and a long standing bug just to add a further curve ball.
First thing to remember is that the system keeps track of which zones have been used for both anytime and off-peak caps. With all day caps in zones 1-6 (unless a railcard is added) this rarely makes a difference. Then there’s the oddity that for some weird reason all bus journeys are treated as off-peak. This came in years ago when weekly bus capping became a thing on Oyster, and it’s never been fixed.
So, your first bus journey added £1.75 to both caps. The tube from Hatton Cross added £3.20 to the anytime cap and set the anytime zone limit to 5. The tube from West Brompton added £1.35 to both caps and set the off-peak zone limit to 4. The 11:31 bus hit the zone 1-4 off-peak railcard cap of £8.50. The tube from Earls Court extended the off-peak zone limit to 5, and the final bus hit the daily bus cap as you said. Had you made another train journey this would have capped at the zone 1-5 off-peak cap of £10.15 (unless you went beyond zone 5).
Thanks for that challenge.