Lets add a little to the above post. He is right, but he left out something.
AMD and Nvidia always make what is called a reference card. This is a complete card including the GPU (Graphics Processor Unit), memory and so on for each model of video card they offer. And they sell these to their partner companies. The partner companies he listed above, can sell those reference cards, but they are also free to buy just the GPU, and make their own unique circuit board to mount the GPU and the memory and all that, which allows them to customize the cooling solution on the video cards.
So when you see something like the Asus Strix, those are custom made cards. Take a look at this page I link below, and it will give you a good example of how these third party companies create unique versions of the same card. Cooling Solution and Speed of the GPU are two of the ways these partner companies can distinguish themselves in the marketplace.
http://www.asus.com/us/Graphics_Cards/STRIX_Series_Products/