Ta, I'm never likely to fly a supersonic aircraft in practise, but it's nice to have my basic assumptiosn confirmed in case the chance ever crops up ;-)
If you have the patience and inclination, back to compressibility error.
The pitot is reading dynamic pressure, but as you go faster, compression affects the dynamic pressure experienced in the pitot, and this has to be factored into the reading.
Or more technically, Bernoulli's equation assumes an incompressible flow, but as air is in fact compressible using the Bernoulli equation to deduce airspeed requires that compressibility be taken into account.
Is that even close to an accurate description of compressibility error?