TL;DR; is that instead of just having full forward/reverse current or not on each winding, the driver can also do things like 90%/10%, 80%/20%, 70/30, etc. current for the two windings to position the motors at angles between the full/half step positions. the actual ratios aren't quite this simplistic, and are designed to achieve something close to constant holding torque and even step spacing, but since the exact angle achieved for any winding current ratio depends on the motor design, and application, it's more or less impossible to get microstep positions that are a precise fraction of the full step angle for all motors/applications. this is mostly OK because 1) you're not making the full step positions any less accurate, 2) you're still making motor motion smoother by taking smaller steps.
a longer explanation can be found at https://hackaday.com/2016/08/29/how-accurate-is-microstepping-really/. if you want to go deeper that that, there are lots of motor controller app notes out there with all the gorey engineering details.
blogodex = {"idx" : ["electronics", "stepper motors", "microstepping"]};
