Basically, the actual processor on the card is made by Nvidia and sold to card manufacturering companies like Asus, Zotac, ect. Those manufacturers will then build the rest of the graphics card around that processor. Most of the card will be very similar between manufacturers, as the processors require very specific card designs but some layout details, cooling, overclock specifics, and appearance can all be changed by the card manufacturer. As well as that, different manufacturers will have different design and quality standards, and different support systems.