Hi Jacek,
It’s going to be a combination of a zonal cap (1-2, 1-3, 1-4 etc) and an extension fare for each direction from that zonal boundary to/from Windsor. I’m having trouble understanding how they are working these out, and I’m waiting on a response from TfL after a trip I made to Marlow. I think it’s going to be an error because the cap should be £19.20.
If you click on the link in your journey history relating to the return journey from Windsor where it says “Contact us about this journey” you could ask them to explain how the cap was calculated, and whether there were extension fares involved.