Namely because Nynaeve's POV reports something otherwise when she faced Moghedien in The Shadow Rising.

Moghedien sliced Nynaeve's hardened air attack, and then replied with a complex weave of Compulsion. What is noteworthy is that Nynaeve reported feeling ecstasy and a desire to serve BEFORE she could slice the flows. Slicing only takes a moment, being very simple, and even if Nynaeve's isn't that skilled with it, the fact that she reported the weave affecting her before she had a moment to slice it implies that the Compulsion weave is not straightforward like a fireball, and it is possible to be ready to counterattack and have your defensive measure fail because you are affected by the Compulsion weave.

It is possible that the actual weave sets the conditions for the subjects mind to be receptive to hypnotic suggestions before the flows themselves settle around the person's body. This may be why "Holding the Power protects one from Compulsion" - it may not have anything to do with some inherent protection channeling gives, rather than the possibility to recognize what the weave is and lash out or slice it before it is given a chance to settle.