I’ve been told on numerous occasions by Boeing and airline instructors that “altimeters will always show the same, no need to compare them”.
Yet, I’ve seen myself a difference of ~100ft between Capt. and F/O altimeters in cruise, which resolved itself after a few minutes...
My guess is that the system uses voted data from 3(?) independent sources. However, if one of these sources fail, then I suspect each side uses one of the two remaining independent sources.
As you already mentioned, Boeing didn’t bother to provide an explanation to us, pilots.