well its not comparing the actual performance of these cards at all, just memory bandwidth. So you end up with the gtx590 lower on the chart than a gtx580 even though a gtx590 is nearly 2x as powerful as the 580. Basically the list is all over the place and it will only confuse people. Its like saying a certain car is faster just because it has a larger engine with more cylendars, not taking into account, weight, traction, gear ratios, forced induction, actual measured horsepower at the wheels etc. The only way to tell a video cards power is real world benchmarks as shown here http://www.anandtech.com/bench/GPU/88
even that is to be analysed as different cards perform differently in different scenarios, resolutions and image quality settings.