Yeah I think this is most likely. Then when you change the serial you're using the calibration data for the wrong screen, but most of the time you're lucky and the two screens behave close enough that an average user doesn't notice.

Probably for touch the effects are negligible as the accuracy needed for touch is much more coarse compared to accuracy needed for Apple Pencil drawing. So if calibration is perfecting things at Apple Pencil-level accuracy, it should already be below the threshold of touch-based highest acceptable margin of error anyway.

Touch and pen would generally require completely different calibrations because touch is detected by the capacitive touch screen whereas the pen position is tracked inductively.

However, I wonder why the pen calibration data is not stored on an controller on the display panel? That would absolutely make sense.

Can it be like:

Pencil also has a small calibration table and when paired with a new iPad it checks and downloads that immediately.

If not, it might also be (more likely) sending whatever it senses to iPad and iPad processes that raw input with the calibration data to determine the final output.

It only makes sense from the perspective of screwing third-party repair and then trying to come up with something with plausible deniability. Are people swapping screens frequently enough to justify the extra complexity of keeping calibration data segregated by serial number?

Printer manufacturers have been pulling the same trick with storing (approximations of) ink levels in chips in the cartridge, claiming that it makes it easy for users to swap cartridges and continue to have accurate quantities and in that case there's a little bit more truth to the argument, but not this one.

I'm just speculating but the accuracy needed for Pencil to work properly is, by nature of the application, much higher than estimating ink in cartgridges.

I mean, if cartgridges' calibration drift a bit it might also be more acceptable for many, but if Pencil starts drawing incorrectly many artists would be extremely frustrated and move away from the ecosystem.

The fact that they showed how using the controller chip from the existing screen causes it to start working correctly shows definitively that this is not about calibration at all.

Unless the calibration data is stored on the controller chip.

But the physical screen is a different screen so any such data is totally wrong.