I've noticed that when flying at high altitudes in mach mode (like a good boi), the conversion between my groundspeed and true airspeed convert to be a slower mach number than what's showing as my mach speed.
For example, right now in the FF 757, I'm holding a .83 knot cruise. My GS is showing 465 knots.
But 465 knots = mach .68.
Why?