I have both a Nexus 7 and a Nexus 4, and I have been playing with the inbox chargers and cords which came with them.
The N7 has a 2 A charger and a short cable, while the N4 has 1.2 A charger and a slightly longer cord. Both cords appear to be the same external caliber.
The N4 charges fine with any of the charger/cable combination. However, the N7 charges well only with its native charger/cable. For example, with the N7 charger/N4 cable, it won,t charge at all when being used, and took 8 hours to charge 50% when off.
The small difference in cable length can,t account for this huge difference. I would be interesting to see others' experiences with 3rd party cords.