Pretty much usually they have different manufacturers which have different models for the same GPU (the chip that is doing all the graphics stuff), which means that even if it is the same Nvidia GTX 750 Ti chip used, they may vary in performance by a few bits (+- 5 FPS gaming wise). It all comes to how the manufacturer (Gigabyte, Asus, MSI, etc) put them together and what cooling solutions they use (different heatsinks, fans) and also how they configure the stock settings (some put higher memory frequence, some put higher memory bandwidth, higher GPU clock but it doesn't differ that much from the original chip that is specified in Nvidia homepage). It is possible to OC almost all of the cards manually, so it doesn't matter that much...