That Myrddraal are simply more human-like throwbacks, which are more influenced/twisted by the TP. That would explain their special powers and why the 13+13 turning trick works.

It was reported from a BS signing: "Myrddraal are not Trollocs who can channel. Their powers are totally independent from the One Power. They really are just throwbacks to the human stock. Harriet added that there are also animal throwbacks, but they just die."

And by RJ:
"The gholam --- singular and plural are the same --- were created, not bred. Supposedly their creation involved making them so that they would be obedient to the Chosen, whoever they might be at any given time. This was an attempt at copying something that had turned up in Myrddraal, which seem incapable of disobeying one of the Chosen, possibly because of the use of the True Power in creation of the Trollocs, the parent stock of the Myrddraal."