The actual race distances at Fakenham appear to be far longer than advertised, so Dave Edwards may be having to make some "additional" fudges to the normal RP method so that more realistic figures come out of the maths. (Taunton, Sedgefield, Kempton, Carlisle, Uttoxeter etc have similar dubious race distances, based on race times)
The basic method has been summarised in the Systems section.
I’m afraid at this time of year in particular, the usually reliable Topspeed going allowances and the resulting ‘time-based going’ are (as robert suggests)guesstimates rather than the usual logically derived estimates.
This is inevitable as many jumps courses change their configurations and race distances on an ad hoc basis to suit the climate and the ground, probably with good justification.
As a result, Topspeed figures are often questionable for jump racing whereas they are much more reliable on the flat.
Exactly what the ground was like on a particular day at a jumps fixture is therefore debateable, to say the least, and cannot be deduced conclusively from the race times.