Just checked the single fare finder. West Drayton to Paddington is an example of an oysterised before 2010 and the discount applies to off-peak fares. New Cross Gate to West Croydon is now Overground and the discount still applies to off-peak fares.
The WDT-PAD example is indeed a little odd since before 2010 the fares were in line with LU fares. Since the introduction of PAYG oyster on NR the fares were frozen in line with LU fares, whereas a journey from LST (underground) to ECR (National Rail) falls under the new fares regime which charges a premium for services that are NR-only or NR+LU.
So WDT-PAD for example comes out at £4.20 or £2.40, (the same fare as Heathrow (LU) to Holborn (LU)).
However a journey from Liverpool Street (LU) to East Croydon (NR) will be charged at the new NR+LU fare (£5.00 or £3.70).
Interestingly out of the two examples above, both will have a NR discount applied, but not if the WDT-PAD journey was extended to say, Holborn.
Incidently a journey from Liverpool Street to East Crodyon might just as easily be completed via ELL from Shoreditch High Street, in which case the fares would be £3.90 and £2.60 peak/off peak respectively, and the £2.60 will have a railcard discount applied to it.
Well, thanks to the legacy Oyster PAYG implementation on FGW services this is turning out to be as clear as mud isn't it?
TSM