That number is the fraction of a step variation you might see from one step to another for an unloaded motor. Yes, it is due to magnetic inhomogeneities, and could be quite irregular around the full circle. Note that since a full circle add up to exactly 360 degrees and (usually) 200 full steps, for every step that is too big, there have to be small steps to make it all add up still. They can't all be 102%, since the number of steps per full circle is exact and constant.
Note that I use this principle to calibrate ultra-high precision x-ray diffractometry systems. With no external standards, and a highly repeatable (bot not accurate) angular scale, one can use 'circle closure', which is this property of cancelling errors, to define angular scales of truly extraordinary accuracy.
See, for example, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911639/
which is one of the papers I have published on this technique. The technique has been used since the 1800's (!).