Just out of curiosity what causes the unit to need recalibrating as it never leaves the car?
Good doggone question. After many years of fiddling with TT units, it was finally necessary to make a change to my GO 740 Live. My 'antique' GO 720 still retains its calibration without issues. The 740 had a bit of a vertical shift that was finally becoming frustrating enough to dive in and try a cal.txt file.
I can tell you WHAT happens, but I have never researched the technology closely enough to tell you WHY it happens.
Touch screens come in two primary flavors... resistive and capacitive. The resistive screen works with pressure on layers of the screen itself, and the capacitive screen works with the presence of an external 'object' (you).
The resistive version is made up of two VERY thin layers of material (one that is metallic, would you believe?) so thin that most of the light from the LDC display makes it through the layers -- else you wouldn't be able to see the display underneath. Where you press on the screen creates a different resistance to occur in both X and Y between the two layers on the touch screen, and that resistance in conjunction with a very carefully controlled source voltage is used to create different inputs to the processor. The voltages that the processor sees with a press determine where it thinks you pressed on the screen.
What happens to the resistive version is that the relative resistance of the material itself changes over time, causing a slight voltage shift in the output. Unfortunately, that voltage shift is sensed by the electronics as a difference in position for the touched point. Why the resistance changes is the mystery that I haven't tried to understand (yet). So what you are entering with the cal.txt file is offset information that 'shifts' how the processor interprets voltages from the touchscreen, correcting the drift in resistance that seems to occur over time for certain units like yours and like my GO 740. Seems only a fairly small percentage of units are impacted by this, and as I say, I have no idea what causes the resistance to drift over time.