I don't think thats true for all of them. For instance my 720 needs 1.44 to be remotely stable with a 600Mhz OC. I think they measure them to where they should be anymore and not undervolted. But more so it all depends on the chip as no two chips are the same.
amd and intel DO set chips above what they need for the bulk of them to be stable, its a smart move, it means they can get higher yeilds of chips they can sell for higher prices.
I have seen chips come out that you can undervold DRASTICALLY (Sub 1volt) at stock, where others of the same model couldnt be undervolved by much at all, BUT they could be taken down a tick or 2 minimum.
also they have to rate the chips higher then the chips really need in order to keep OEM's from cheaping out and using boards that have voltdrops below what the chip needs to be stable at stock, again smart move for volume sales, and its kinda fun for geeks as well when u can oc and undervolt a chip(if u get a good one)