I see no reason why Rahvin would say he matched well with Sammael if he was actually stronger!

And you're making too much of the levels. Graendal can face down Rand, according to Rand himself, and she's weaker than him.

I think these upper strength levels are not so far apart. Yes, there is a perceptible difference between the Forsaken, but even the weakest can be an honest contest to the strongest. None of them are negligible.