Thats what I had in consideration i think its called binning (transistors) but I believe that they also add some safety margin just in case. Thats why you can always overclock instead of having the oveclock preapplied (notice that it wouldnt be called overclock then) AND that safety margin is the MHz that enough cards (in their opinion) work with that MHz
The new cards do get reliably higher hashrates, but only at certain special frequencies such as 1500 or 1800 RAM on 280Xs coupled with 1080 or 1160 core respectively for 750 and 780-807 khash!