The cockpit gage always shows the mechanical speed, not corrected (i.e. performance-related) speed - because the certification authorities want you to know how close you are to the max certified rotor speed. (Today that's not much of a concern to pilots; most engines have a triple-redundant overspeed protection system.)
BUT - in the very early days the gage expressed actual rpm like a recip engine. Sometime in the early 50's it became standard to express this as % rather than real rpm, and that's how it is today.