Plus, every video card is slightly different. The advantage of buying overclocked ones from the factory is that they are guaranteed to run at those higher clocks, meaning they were at least somewhat cherry-picked. They are possibly more likely to run at even higher overclocks if you're so inclined.
This is not true in more cases than you think. For example, the EVGA SC cards are BARELY OC'd - so it's just a different BIOS. I guarantee there is no speed binning there. Whereas cards like the Classified - yes, they're binned.
Example, for Titans - they "test" versus bin: http://www.evga.com/forums/tm.aspx?m=1915532&mpage=1.