Binning.
RAM that clocks well will be in the Dominators usually.
I find Corsair tends to bin very aggressively, so buying less than their best tends to mean you shouldn't expect much in the way of overclocking, though that's obviously not true every time.
Luck will play a part as well.
You could also check what ICs are being used currently at their HouseofHelp forums...i've no idea, since they switch ICs so much.