I think what is missing from the "Vehicle A is travelling at...." scenarios is the vital phrase (and one which is always included when these test results are reported by those responsible for conducting them) is "all other factors being equal". In reality, "all other factors" will never be equal. Numpty driver in Vehicle A, who has his brakes serviced by Qwik-Fitt Spanner-Monkey who used cheap parts and didn't give a monkey's uncle about the quality of his work, is going to produce different results than a driver in Vehicle A.1 who uses the best brake parts he can reasonably afford and competently fits them himself, doing a top job in the process. So if Vehicle A has so many variables (because, once again, "all other factors" will never be equal), then the result can't even be compared properly to Vehicle A.1, let alone Vehicle A at 30MPH and Vehicle B at 40 MPH etc etc.
In a nutshell, just as the safety gained by the 10MPH reduction in speed can be lost easilly by other factors (tyres, brakes, shocks, driver skill, driver awareness at that precise moment in time), the safety lost by driving 10MPH faster can be regained by other factors - good brakes, good driver, etc etc.