PRM specification is lab tested "no load" RPM. the specification is based on the motor spinning RPM + gearing multiple. Measuring with loading (like drill bit/backing pad) will inevitably lower measured RPM.
Since polsiher backing pad is larger in size (compare to drill bit), when spinning such larger size disc, the "Angular Inertia" is way higher and further lower the spin RPM due to conservation of Angular Momentum.
A good analogies is that, the spinning of figure skater. When they spin, the spinning RPM is higher when arms close to body. PRM is lower with arms spread.