SR-71 flew 2193 mph. How can this mathematically be Mach 3.3? Virtually every time I read about the Lockheed SR-71 (including other threads on this forum), I see that its speed record is listed as approximately 2193 mph.  The same blurb will invariably say this is "about Mach 3.3".

But when I search for how fast Mach 1 is, the number that almost always comes up is 767 mph.

Now 767 * 3.3 comes out to 2531, well above 2193.  If I take 2193 and divide it by 767, I get Mach 2.85

I doubt I've stumbled on something new here.  So what am I missing? How can 2193 mph be Mach 3.3 when 767 mph is Mach 1?
The SR-71 was only capable of Mach 3.3 flight at altitude. You can see the limitations in the pilot's operating handbook(POH) for the aircraft:



At higher altitudes the relative speed of sound is lower. If we use this handy NASA calculator in a standard atmosphere the relative speed of sound at 70,000 ft. is 660 MPH making your 2193 MPH equal to Mach 3.321



Its also important to note that the word "about" is often used as the SR-71 was flown to a maximum compressor inlet temperature(CIT) not a maximum Mach when doing a 3.3 run. According to the POH


  Mach 3.2 is the design Mach number. Mach 3.17 is the maximum scheduled
  cruise speed recommended for normal operations. However, when
  authorized by the Commander, Speeds up to Mach 3.3 may be flown if the
  limit CIT of 427°C is not exceeded.