I was waiting for more details before replying, but I’ll offer some thoughts now.
What you need to remember is that each time you touch out the system tries to calculate the fare from the current origin to the place where you’ve just touched out. So, what you ended up doing looks like it should have been:
Gatwick Airport to West Hampstead Thameslink: £7.10 (journey ended)
West Hampstead Thameslink same station exit: £7.00 (max fare will be cancelled by entry within 45 minutes)
West Hampstead Thameslink to Willesden Green: undefined (2 leg journey starting with OSI)
£1.90 is a TfL-LU zone 1-2 off-peak fare with railcard which seems logical as the cheapest potential route would be via zone 1. Starting (or ending) a journey with an OSI is troublesome if the fares don’t exist publicly, or do but are not what you’d expect.