HDD's are NOTHING at idle (or when spun up for that matter, several watts). Your GPUs however, compared to the 7 series, dont drop to a 3W idle state, so I would imagine its that, the mobo itself, and the CPU
Last I checked the 6950 wasn't a 7-series card either. We're talking about ASRock's computer that has a 6000-series card in it, not your with your power sipping 7-series card.
The point is that despite the numbers being low, that's not all the CPU and there are other components that use power and even if you say those numbers are low, low numbers add up pretty quick when you're usage isn't a whole lot to begin with.
A hard drive consuming 7 watts on a machine that draws 200 like mine is nothing, but on a computer that draws 70 watts at idle (I'm assuming the drives aren't spinning down,) that 7 watts just went from being well under 4% of your power usage to 10% of your consumed power. So the ratio of those smaller usages impact you more because the number is already so low.
Also spinning up and slowing down drives a lot on a regular basis actually puts more stress on the motor in the drive. I've had the best luck leaving drives spun up because I'm perfectly willing to use the extra 25 watts to do it. (I'm rounding, I suspect 7200 RPM drives use more then 5400 ones like where i got the numbers from.)
The only real point I'm trying to make is the lower the draw, the more other components can impact that usage, such as add some hard drives or adding a PCI-E expansion card.
We already know how low it idles and that isn't in dispute, it's just the method.