Chris,
Your spindle is open-loop, right? i.e. - you do not have a spindle speed encoder? I believe the "RPM" DRO will only be active if you have a spindle encoder, and it will show the actual spindle RPM, which is not necessarily exactly the same as the commanded RPM.
I have a sort-of similar problem with mine, and I also have a SmoothStepper. At low spindle speeds, when I turn the spindle on, it comes on too fast (sometimes 2X), then over a period of several seconds settles down to the correct speed. Mine is open-loop - no encoder (yet).
Regards,
Ray L.