At fl 370 the speed of sound equals 573.57 ... So if you round up your Mach number from .755 (Tas 433) to .764 (Tas 438) there is a 5 kts difference but still flying Mach .76
... If you use 573.0 as a reference speedd of sound your Tas will be 432.6 and 437.8 respectively ... Maybe the target during the flight tests was an accurate n1/fuel flow / SR and the Mach number the resulting value of this target
With a heavy airplane you should(most probably) fly .764 to have a stable airpeed and with a light .755 in order to achieve the best range
(always flying .76).Always talking about a constant Mach number table .. Right ???