Out of curiosity, I just ran a quick comparison. I think your right about the cable and connector quality, but maybe it's not a length issue.|
I happen to have ordered my BPi from the same seller, so I also got the Quatpower PSU (which I'm not using atm since it's not working reliable) and a cable that came with it. I never unpacked the cable actually, because I had a spare one which is slightly longer. Anyway, for the test, I unpacked the cable and compared it to the longer one I already had (came with a Logitech device).
The result for me looks like this:
When I use the Logitech USB cable, the voltage during idle is is around 4.95V. When I replace the Logitech cable with the cable that came with the Quatpower PSU, then the voltage during idle is 4.75V. So there is a noticable difference, but in my case it is not due to cable length. In both cases I used a 5V/2A power supply from Asus (came with a tablet computer).