Well, by meter alone, if you're running Class C, that would be about right - low Dead keys, if you think of it this way, are low volume signals arriving into a stage - the amplifier is just sitting idle - so there's noise in all of that (the THD IMD stuff) - so as you improve the signal transfer into the amplifier, the waves it can generate have less IMD but the THD stuff will always be there.
The Class AB stuff - the better the bias that is there - the better the signal transfer into the amp (I'm referring to the admittance) so as I can see, the AB bias kinda "Washes" out the low-level signal but as you swing power into the amp stages, it gets cleaner because it "sees" a more accurate signal to amplify.
So do you do fish, or poultry?