When I first got my TB20 in 2002, I too was puzzled by the two tables.
I asked myself "why would anybody fly at best-power doing say 140kt when they can fly at best-economy also doing 140kt, and use less fuel".
Neither Socata nor the dealer (Air Touring) were able to answer.
I now think that Socata just pulled those figures from the Lycoming performance charts e.g.
this or
this.
I am 99% sure they never got them from actual flight testing, because I am very very sure that the original test aircraft were not fitted with precision fuel totalisers... why should they be (years before the instruments became common in GA and about 20 years before they became a TB factory option... and when they did become a factory option they stuck the transducer in the wrong place where flow turbulence caused a 20-30% error and that is how they shipped them
) when Lycoming provide you with a nice table which you can just plug into the POH and get the whole lot certified as-is?
The upshot of this was that the POH data was probably accurate from some 1960s Lycoming engine bench tests but was almost useless for working out the real range of the aircraft in different conditions.
Fortunately, by judicious operating practices, and with an accurate flowmeter, I can now get some 20% more range out of it than the POH and the sales brochures suggest.
My advice is to take POH data with a huge pinch of salt. And if you want to fly real distances, get a fuel totaliser installed. It will be the best 1-2k ever spent (alongside TKS prop de-ice
).