"Flat" increments "number" if and only if it has a separate post box. If you have several flats behind the same box in a common front door - a common set-up in the UK when a larger house has been converted into several flats - then as far as the postcode system is concerned, those flats don't exist; since people still write "Flat 3, 11 Wisteria Drive ..." on letters this creates various issues with denormalised addresses.
Not always, I've had that exact situation and the flat number was in all systems. All houses in my postcode had several entries for flats and as far as I can tell, they all only had one box.
