Graendal is of very rare strength and therefore one of the stronger Forsaken women. Therefore she is one of the few Forsaken who exceeds Nynaeve. Nynaeve is stronger than Semirhage in all probability according to Brandon. Someryn from her quotes is probably one of the six because of her quotes on her strength, but she implies that Graendal exceeds her.

So there, that's my short story on why they're separated by THREE LEVELS ... what makes you sure that Semirhage is strong in the Power except for her being respected - a fact which I find flawed in any case.

I don't understand you, Shannow. It's like you can't comprehend that someone like Semirhage might not be close to top strength in saidar. If she was as strong as Moiraine she'd still be a legendary torturer, you know. It's her craft that makes her deadly. No one has ever referred to her strength. Even Lews Therin said that her reputation as a torturer loomed so heavily that he forgot how dangerous she was otherwise. Aginor is immensely strong in saidin, but he's a weak personality and Graendal mocks him, and Demandred. I've never found strength extrapolations like this to be useful.