From The TH web site:

DLE-55RA "Output: 5.5 hp @ 8,500 RPM"
DLE-55 "Output Power: 5.5 HP at 7,500 RPM"

Both have the same power potential but the higher band of the RA model will allow for more speed. The trade off will be less bottom end torque for a slower spool up into the power band. As far as why? It has to do with the different porting of the cylinder. Changing the porting on a 2 stroke is much like changing the cam and valve size in a push rod engine. The new design allows for a higher RPM peek but obviously gained no "extra" overall power in this case. Anything more than that would just be guessing so I will wait fore a DLE designer to chime in to explain.