Also, if you go to 3dmark11 and search for results you will find that depending on the person who submitted the result given the exact same system you will find that one person may have a really high score and one has a really low score.
GTX580 is faster if the benchmark can only use 1 GPU.
That is because the GTX590 is actually clocked slower than the 580 to meet thermal/power requirement specs.
This is also the reason why 2 GTX580s in SLI will beat a single GTX590.
Also, why don't you just use the GPU bench here on tomshardware?