The problem with the in-out-in manoeuvre is that you have to be aware what the fare to the second station (or from the first station) is because that is what will be charged.
Gatwick to West Hampstead Thameslink is £10.70 which will be the intermediate fare charged when you touch out there. Touch in at West Hampstead LU will continue the journey, then touch out there will end it as a Gatwick to West Hampstead LU journey and the charge is then £13.50.
If you’d done the out-in-out manoeuvre at West Hampstead Thameslink then the charge from West Hampstead Thameslink to Willesden Green is undefined. It will charge something, probably including zone 1, but isn’t shown on fare finders because it’s deemed an illogical journey.