I'm trying to book a cash double open jaw OSL-LHR-JFK/SFO-LHR-PMI in March and April, and I'm struggling to understand why it's pricing the way it is.

In particular, I want to take the evening flight (BA286) from SFO. EF shows A availability on both the afternoon and evening flights:

And the two fares I want to combine are, per matrix, `ANN4C8S4` and `ANN6C1S4`. However, matrix only shows `ANN6C1S4` as open on BA284. For the evening flight the booking engine only offers me `F1N0C9S0` which is about US$5000 more. This is despite showing A availability on LHR-JFK/SFO-LHR, and the connecting flight to PMI being the same in both cases. Availability/pricing on matrix agrees with google flights/BA multi-city search as well.

Any fare experts here who can help me debug what's going on?