If you grab a distributor, you cannot turn the rotor and the drive gear or reluctor independently of each other. they are on the same shaft. The rotor appears to move in the timing light/hole-in-the-cap test because the only time you see the rotor is when the light fires(strobe). The light fires at different times because the timing is changing.
Hook up a mighty-vac to the vacuum can and pull, the plate moves the pickup relative to the rotor and reluctor. This is moving the same plate that the mechanical advance moves.
Reluctor phasing is important because the point where it triggers the pickup has to be when the rotor is in-line with the contacts on the cap.
If the rotor is truly changing position, it's because the drive gear is riding up, out of the oil pump as rpm increases.
What am I missing here?