Contactless has seen that :
* the Z1-2 cap (7.40) plus extension fares (3-6 for coming in from Slade Green is 3.05) plus the 3-6 single Stratford Int to Slade Green, with the buses covered by the cap
is cheaper than
* the sum of the individual fares as you’ve listed
so you get the cheaper amount. The difference will (eventually) be given as a refund to the Oyster card, assuming it’s an undiscounted adult card.