I'm sure the Radeon HD 5870 wasn't the fastest single-GPU card for six months. I'm also sure the HD 5970 was slower than the fastest NVIDIA had until the GTX 590 was released, the GTX 295 and GTX 480. Speaking of the GTX 590, I'm sure the HD 6990 was slower too and they didn't have the same performance.
AMD hasn't been able to compete with the cards below those, either, when it comes either to performance or price. The HD 4850, 4870, 5670, 5770, 5850, 6790, 6850, 6870, and 6950 NEVER existed. I just made them up.