Every layer is real data from your files, drawn on one map. Switch layers on and off. Nothing here is fitted to the arc.
The 7th arc is the locus of positions at the final handshake (00:19 UTC) from your reference file. Candidates are Phase 5A points near it (65 to 1,306 km away). The density is the combined reverse-drift distribution from the baseline run with assigned classes. It is a different kind of quantity from the arc and candidates, and it depends on assumed find coordinates, the class table and no explicit Stokes.
The density peak (35.25°S, 55.75°E) sits about 870 km inside the 7th-arc ring and 918 km from the June BTO-only endpoint box (43.5°S, 55.5°E), which is on the ring. The two share a longitude and differ by about 8° of latitude. That is as far as the data goes. The drift side does not locate the aircraft. Task 9 (complete) found the same broad western focus in the shuffled-region and random-coast nulls, so the peak is not specific to the finds; its cause is the open question, and Task 10 is set up to isolate it.