quote from article:
"We first observed differences between the Radeon R9 290X cards that AMD sent out for review and the ones being sold online just before our R9 290 coverage went live. After additional testing, we have answers, feedback from AMD, and more questions.
Leading up to the Radeon R9 290X launch, I had two cards in the lab, both supplied by AMD. They ran at slightly different core frequencies in single-card configurations. No big deal, right? That was just PowerTune doing its job. But paired up in CrossFire, even with plenty of space between them, heat build-up forced the pair to drop to even lower clock rates. Right away, I was able to establish that AMD’s Hawaii GPU operates within a range of clock rates, determined by a number of variables. For R9 290X, that range starts at 727 and ends at 1000 MHz. As an aside, I do have an issue with vendors simply advertising this as a 1000 MHz GPU."