I read somewhere that the reason why they are better is the notch in the ground electrode. Sparks like to jump from sharp corners.
The way the SOHC coils run, the current flows through 2 plugs- "down" one plug from the secondary winding, through the case, then back "up" the other plug and back to the secondary. In the case of the plug where the secondary current runs "down", the spark jumps from the center electrode to the ground electrode, as conventional sparkplug design intends.
But, for the plug where the current goes "up", the spark is trying to jump from the ground electrode to the center electrode. The broad, flat face of the ground electrode is not the ideal surface for spark propogation, although it does work. The groove in the ND ground elecrode offers a better condition for the spark to jump. This is most beneficial at idle speeds, when coil voltage drops along with alternator output.