I am Southern Africa-based at the moment. Just as I was planning a work trip to London, another bit of work came in requiring me to be in New York just beforehand.

I was very surprised to see that by adding the LHR-JFK-LHR sectors to my JNB-LHR-JNB trip, the increase in cost was only £550, with all four sectors in First. Is this a known thing? Am I missing something?

JNB-LHR-JNB is around £3,800 in F.
JNB-LHR-JFK//JFK-LHR-[two-night stopover]-JNB is around £4,350, all in F.

These are for mid/late-August dates, with quite a few options available. I decided to force a very long connection at LHR on the outbound, so I could have a CCR 'meeting' with a colleague, but I could have also added an overnight.