Hi Eric - Steppers move from "magnet" to magnet and have a mechanical accuracy. Typical commercial motors are +/-5% per step non accumulating. You can get 2% motors. So for a 200step motor ie 1.8degs per step you have a 1.8*0.05= +/-0.09deg accuracy or "zone" that the motor will stop at, at each full step. So you can now figure out the "zone" the motor should stop in mechanically if you know the lead of the screw (then there's also backlash to consider and hysteresis which means you will get a different measurement going one way vs the other). Then you have the electronic accuracy. Microstepping is a way to sort of balance the motor between mechanical steps. This has an accuracy as well.
So there is an averaging going on as the motor moves it stretches and compresses slightly around each technically correct spot (mechanically and electronically). Over many moves it averages to correct. Cheers Peter S