There can be many reasons why the ASI does not read correctly provided the ASI has been properly corrected in the first place.
a) blockage in the pitot tube, this doesn’t have to be a huge blockage
b) static system blockage. Again doesn’t have to be a total blockage
c) a turn if the pressure on the static port changes because of the turn
d) several light aircraft including many ULM have their static port inside the cabin and most aircraft that have external static ports will have an emergency static entry on the inside of the aircraft. Usually POHs or AFMs have a note that says when the emergency static port is used there is likely to be a change in pressure and therefore a change in ASI. It also often notes that this difference can be as much as 5kts
e) So because air pressure is different at different parts of the aircraft the same ASI would read differently which is the whole reason you calibrate the ASI on the aircraft.
All this presupposes that the instrument itself is in good order. ie no holes in the bellows or sticky needles.
But could this be a problem in the case the OP specified. IIRC his problem was that his TAS remains only slighly above his IAS as read on his ASI/PFD at temps above ISA and at altitude.
On most aircraft it is the same static and pitot ports that feed both the ASI and do the calculation physically or electronically. So it is true that the ASI has been calibrated incorrectly and is giving a higher than correct IAS.
This would certainly give less of a gap between IAS and TAS IF both speeds are given by or through the same instrument.
GS on the GPS has little to do with this, however, it is very useful for approach speeds of you cannot trust your ASI.
A simple check of coherence across a range of altitudes and temps between ASI reading and TAS reading will show if there is a problem and will give some indication of what the problem is. After that it is more likely to be an instrument problem than a problem with the positioning of your pitot or static ports. After all this aircraft has been flying a long time.without problem.
Now true altitude is a much more dangerous problem.
