Nvidia always makes the chip. As for the card, it depends. When a new line first comes out, the card is manufacturered by 3rd party subcontractor, then sold to distributers such as EVGA, who puts on different labels. It all comes from the same factory floor, no matter the brand. There is no difference whatsoever. Right now, we're at this stage.
Later on, distributers begin to produce cards in house or through their own subcontractors. The gpu will still be the same, but minor parts, like caps, might be from different sources. It still runs off the same reference design, and there should be no real-world performance difference.
At the last stage, when the cards begin to become outdated, some distributers begin to develop nonreference designs. Those will look physically different, and can perform differently. For example, Asus's nonreference version of the aging 3870x2 supports 4 monitors instead of 2, and Powercolor's version use ddr4 instead of ddr3 vram.