To account for in flight movements and uncertainties in the pax distribution, both regarding seating and passengers who refuse to conform to the standard weights, the CG envelope is curtailed. Chances are this is what you are seeing. A different configuration could mean different sectioning of the cabin which would in turn mean different curtailments of the envelope.
Edit: BTW, the the aerodynamics and flying qualities have a larger impact on the CG envelope than the limits set by the structure.
Regards,
Fred