Well , I just reviewed some pretty informative sites regarding error on instrument readings and actual ambient conditions . Looks like there are a number of compensating and correction setups. Temperature etc.. I can see how the device is sometimes not showing what we think it is reading. One site was " pitot- static systems and instruments".
I should not have said "pressure compensated" tho effectively that is what happens because if you change temp the pressure changes . A "true airspeed" system which is accurate in all conditions would be a nice bit of work.