taken from link above.
why does purer silicone oc higher?
Edit: also if they do limit the chip artificially, shouldnt all chips in the same range say 950 have the exact same oc.
But simply put, the better quality waffers have properties that allow for less heat dissipation due to a variety of factors. In turn this allows the chipset manufactured based on higher grade material to perform better, i.e. allow acceptable operating temperatures when higher voltages are applied.