Those stats are basically comparing their card - a factory overclocked card to other cards at stock speeds... If you over clocked the other two cards you would see closer to 0% +-5% performance over the GTX 690 and ~100% +-5% improvement over a 7970 (as it should due to the fact that all it is is basically 2 7970's in CF).
Talking about two 7970s, CrossFireX doesn't allow utilization of the VRAM on both cards but this has 6Gb on one card, that is a nice bonus too.
I believe they list it like that to fool people into assuming that it can use the full 6gb. I believe it is really 3gb on each card with each card having 384 bit bus... they just add them up and call them the specs. That is exactly what nvidia has done with their GTX 690 as well.