Why is the miles per hour different for the Mach number for the cruising speed of the Boeing 747?

Mach Number is the % of the speed of sound at which an aircraft travels. E.g. Mach 2.2 is 2.2% of the speed of sound.

The relationship between Mach number and miles per hour changes with temperature (NOT pressure). The higher the temperature of the air, the faster the speed of sound becomes, and since Mach number is simply a multiplier for the speed of sound, the hotter the air is, the greater the number of miles per hour that corresponds to a given Mach number. Temperature generally drops with increasing altitude, so Mach 0.85 is slower at 36,000 feet than it is as sea level. "Std. atm." means "standard atmosphere," which means 15° Celsius at sea level. The temperature at 36,000 feet is far below zero. The speed of sound (and thus the mph equivalent of Mach numbers) is therefore much lower at 36,000 feet. You may be further confused by the fact that airspeed indicators usually show airspeed that is uncorrected for pressure and temperature. Because of this, 250 knots at 36,000 feet is actually much faster than 250 knots at sea level. If you are wondering why the airspeed indicator isn't corrected, it's mainly because this indicated airspeed (as opposed to true airspeed, the actual speed of motion through the air) is more closely correlated with the performance of the airplane, so it's more useful to have than true airspeed, in most situations. For example, maximum airspeeds are more constant when expressed as indicated airspeed (IAS) than when expressed as true airspeed (TAS), and things like drag, stall speeds, and lift are more correlated with IAS than with TAS, too.

Mach is related to the speed of sound, which changes with pressure/altitude. Unless you know and allow ofr the pressure altitude, you can't equate mach and MPH.