Historically this has been the case, but for the past year it hasn't been. The 290x used FAR too much power, generated too much heat, and didn't perform well enough. And then Nvidia brought out the 780ti which used far less power and performed measurably better. And then the gap widened significantly when Maxwell came out and cut power consumption and heat drastically. Right now AMD only controls the mid-end. The 270(x)-280(x).
Now, this may yet correct itself - I've heard good things about the upcoming 390x's performance and power consumption. But we will see.