Okay - need some help on this anomaly.

First rig - Sapphire 7970 standard edition with Vapor X, used settings suggested in Taco's thread - out of the box 650k/H per second.

Second rig - two (2x) Sapphire 7970 "GHZ Editions", used exact same settings in Taco's thread - out of the box - only about 350 k/H per second.

I have spent the last week in "newbie purgatory" so have been unable to discuss this issue.  Finally on the second rig, I was able to get 500k/H per second/card by putting aggression back to 13, and overclocking the memory to 1680Mhz.

Can anyone tell me WHY do two of the same brand/manufacturer, same model, have such huge variance in performance?

Why would the two "Ghz editions" VASTLY under-perform the standard one?

Does anyone have any suggestions experience with a Sapphire "GHZ Edition" that could explain the variance?  The standard one was plug and play @ 650k.   The better ones have taken a week of tweaking to even reach 500k/H, and NONE of the recommended settings worked.  I know it is not bad cards because they both perform the same - what is different about the "GHZ editions" that is hurting them so much, and made them so finicky?