Companies use different kinds of heatsinks on the core processor and RAM (some don't even use heatsinks on the RAM). And, some companies, for ex: XFX, redesign the PCB board to their liking. You will find you have better temps from from manufacturer to another. However, the warranty is a deciding factor for some people.
Personally, I don't care bout the warranty, because if the card fails in a year or 3, I would rather buy new technology.