The attached page is from a document on the XH-59A ABC after a 128 hour flight test program.
The center paragraph in the right hand column is directly related to the subject of your post.
It has been said that the rotor hub constituted 50% of the craft's total drag. This combined with other information on this page could be related to the current subject.
Maybe the lower RRPM for high-speed flight was not actually 'geared' into to the craft. Maybe it was an aerodynamically generated rotor speed.
Dave