Golly, haven't we professionals made this complicated!
chris......
Pilots fly IAS (Indicated Air Speed). This is not the True Air Speed (TAS) which you will see displayed somewhere - TAS varies with height (air density) for a constant IAS (higher altitude = faster TAS).
Mach number, also displayed somewhere (e.g M0.76) depends on TAS, not IAS. As you climb at 320 kt. IAS your TAS increases, and so does your Mach number.
Aircraft are limited by IAS and Mach number, to avoid a) falling apart, and b) unwanted aerodynamic events leading to falling apart. The manufacturer helps us to stay within limits by displaying the lower of the two.
What you have observed is the Mach limit taking over from the IAS limit, due to a/c altitude.
Suggest you find the TAS and M indications next time and observe what happens as you climb/descend.
TP