About discrepancies in performance figures, small changes to the propeller in terms of its condition and the airframe as a whole can produce different performance figures.
In the early stages of testing, Rolls-Royce often experimented with different supercharger gearing in MS and FS which produce differences in performance, especially at which altitude the maximum speed was reached.
At full throttle, the maximum Indicated Airspeed is approximately the same within a certain range of altitude and finding the “sweet spot”, by adjusting the gearing to the altitude just before IAS starts to drop, then the IAS translates into the highest True Airspeed. Then, MS is maintained until TAS drops and the gear change to FS occurs.
The unknown factor that prompted these experiments was ram air pressure due to actual speed and air density.
One example is the Mk.XIV, which is quoted to attain 439 mph at 24,500 ft from one source and 446 mph at 25,400 ft from another and 448 mph at 26,000 ft from a third. These figures are probably not from the same aircraft but indicate that there were differences as mentioned above, probably in supercharger gearing.
The maximum speed is attained in FS gear. At lower altitudes, the differences in MS gear may be even bigger since different gear change aneroids could be fitted to suit operational needs. As I´ve understood it, adapting engine performance by altering the gear change altitude was not uncommon for the Mk.XIX recce missons.
Christer
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