All pickups have a resonancy peak, that is to say a hump where some frequencies are more prominent than others. The centre of this hump and the degree of prominence and width is controlled by the overall resistance in the windings and the voltage of the pickups output. Coil split pickups can have a lower voltage than single coils in split mode, if they're designed to be roughly the same loudness as a single coil when in humbucking mode. That might be why the nordies sound like they lack low end maybe. With a single coil pickup you have one fixed design. With coil split pickups there's a compromise of sorts, either the pickup is designed to make humbucking OR single coil mode sound better.