[awesomeMilou]

But realistically this would mean I couldn't use an iPad with it's pen without connecting it to the internet first. I mean how would Apple bake every calibration for every display into all devices, even ones it hasn't even produced yet?

[numpad0]

It's the display that needs calibration. Pen doesn't know where it is, the screen does. So each complete device or at least the screen needs to have a valid calibration table or a function for raw xy -> calibrated xy.

[mwint]

Because the screen was on the device when it was calibrated.