Nice explanation Keith, thank you.
To understand why some aircraft are affected less than others we need to consider the type of engine used. The degree to which thrust decreases with increasing TAS depends upon the ratio of the exhaust speed to the TAS. In high by-pass engines with their low average exhaust speed each knot of TAS causes a significant drop in thrust. But in a low by-pass or no by-pass engine the higher average exhast gas speed reduces the effect of each knot of airspeed.
I guess there's a continuum from almost-constant-thrust turbojet through low-bypass turbofans, high-bypass turbofans to almost-constant-power turboprops.