Power consumption
There's a price to pay for jacking up the GTX 560 Ti's clocks all the way to 1,000MHz core and 4,580MHz memory. That price comes away of significantly higher system-wide power-draw. The Gigabyte is pre-programmed with a higher under-load voltage - 1.11V vs. 1.025V - when compared to the reference card, which helps push the watts up, too.
Reinforcing the power-draw point, running GPU-bustin' FurMark shows that the Gigabyte SO pulls an additional 81W watts, which, as the graph shows, is more than a stock GeForce GTX 570.
