This reason is I avoided breaking the OSI by means of tapping in and out at the Underground station, because the tap in at the Underground station would reopen the journey rather than starting a new one, and when I tapped out the fare would be adjusted to the fare from Gatwick to West Hampstead (Underground).
However, this record looks like the existence of OSI overruled the fact that a same station exit between 0-2 minutes is cancelled within 45 minutes, which can prove problematic if one accidentally enters the wrong station among a set of OSIs to start his journey.
I also tried entering a station before 09:30, realised that it was too early and exited immediately (where a peak maximum fare was charged) and re-entered after 09:30, and the correct off-peak fare was finally deducted.
I would have used KeyGo for the Thameslink leg if I knew I would travel this route to prevent problems, but unfortunately I was trying to catch a Southern train for Clapham Junction (for Kensal Rise, where a through Oyster fare was cheapest) and the train door was closed in front of me.
