I think that Moridin nailed it on the head in his assessment: Moghedien got caught up in her schemes, like a spider stuck in it's web, whereas Graendal is smart enough to scheme, gain information but recognize a mess as soon as she saw it coming for her and strong enough to choose her battles.

I think Rand hit it pretty accurately, as well: he claimed that he figured out how to defeat a more cunning opponent, but I don't think he realized athat as soon as he was in Arad Doman, he was on her battleground. Hell, this is a man that accepts her being in the palace without even considering the possibility that she Compelled the guy, walked through a Gateway, and stopped maintaining her weave or had the distance attenuate her flows.

Even furthering the ridiculousness, this is a man who has yelled at his friends for trying to prevent him from causing catastrophes and abominations but leaves a servant in his room during an intimate conversation for so long that she could walk right up and collar him.

Moghedien is paranoid but scared; Graendal is paranoid, but she'll leave if need be because that is purposefully why she avoids the type of fortresses favored by the others. This wasn't even a surprise attack - a man that channeled enough saidin and saidar to crack the world in half isn't someone Graendal is unbalanced enough to stick around to talk to.