its just that h-frames tend to twist along the longitudinal axis, because they tend to have skinny center bodies. the h-frame motor rotations are such that a longitudinal twist would create a favorable control moment (instead of an adverse moment).
depends on how bad the twist is (e.g. how low the torsional stiffness). i have no idea what the effect on dynamic behavior is or if there are potential unstable modes. someone else can chime in here.