Umm,
Asus, MSI, and Gigabyte all manufacture both Nvidia and ATI and are among the top selling board partners for each. I doubt the all have two assembly plants.
I'm not sure who owns sapphire or if Pine tech owns anyone besides XFX either.
The leaks that come from across the pacific likely do originate at a partner. It is not much of a stretch to believe that one of the leaks has seen both cards personally. Also, just because a card is marketed by a company does not mean it is manufactured there. For all I know foxcon could be building a massive amount of both cards for a dozen different labels. I'm certain that many partners contract the manufacturing out to a third party anyway. I bet nearly all of the reference cards are made at the same place, and likely that place is a huge multi national assembly company that has no affiliation with AMD or Nvidia and likely makes both much of the time. Probably in the room next to the smart phones, mother boards, cars, and Barby dolls...