If the SF71 was like many other military aircraft, then it could be flown far beyond the long-life envelope. Overclocked, if you will, getting extreme performance for an hour at a cost of years' wear on the engine and body. You can see how this approach might make sense for an SR-71 where one might sensibly want to sacrifice the aircraft to get some pictures out. You can also see how this complicates defining "top speed".