I think there is different components and levels of them in the overall production to distribution process. Its like having one company that makes the VPUs, another small amount of companies that make the chipsets, and then a group of companies that make the entire card. The latter of all are the companies we are commonly familiar with.
But the point is that Nvidia doesnt have a 'built by Nvidia' offering like ATI does, even if ATI doesn't actually build them themselves, at whatever level.