To get the best performance out of a recurve, the recurved ends of the limbs need to be bent almost to the point that the bow starts to stack. Short limbs will start to stack sooner than longer limbs. For a given bow length, a person with a longer draw length may need longer limbs mounted on a shorter riser to avoid stacking, while a person with a shorter draw length may need shorter limbs on a longer riser to be able to get the optimal amount of bend in the recurved limbs.
I'm not sure why a person would want short limbs on a long riser for a longbow. Maybe, as Orion said, for a little more speed?