While not an expert, my guess would be that the parameter that is showing the main rotor´s rpm isn´t correctly read or translated into x-plane parameters.
X-Plane works by using "datarefs", the variables that govern all systems. There is certainly a dataref for main rotor rpm in X-Plane, the question is wether the .fdr format also provides this variable, and if X-Plane is reading it correctly.
I would file a bug with Laminar Research, and since they pride themselves with being an application with a lot of "semi-professional use", I think your chances aren´t that bad that Austin might sneak that fix into the next update. Google "X-Plane 10 bug reporter" to get to the correct site.
Good luck, Jan