Mower deck Blade tip speed.

Recently purchased a z Master model 74444 48 inch deck with a 21HP Kaswaski. Specs say engine speed maxed at 3600RPM (Probably adjusted via governor) and blade tip speed should be 18,500FPM. Measured engine speed at max throttle with a lazer tach and get 3570 RPM. Well within reason for a 2010 z master. Next measured the spindle RPM and get approx 4200RPM, and calculations indicate that the tip speed is 17,500. Toro specs for this mower says tip speed should be 18,500FPM. Somethings not right. Any ideas ?
(The PTO clutch was replaced in 2015, so is there a PTO that will fit a model 74444 z master that is set for residential use vs commercial use ?) IIRC government limit for residential mowers is 17,500FPM and 18,500FPM for commercial...

Mower deck Blade tip speed.