i think it means to say that the board can handle overclock to the memory up to 1600 although it's designed with guaranteed threshold of 1333 speeds at stock settings..
It's not really running the memory up to 1600. It's running the FSB up to 1600. The mem would be at 400 x 4 for the quad data rate on the fsb = 1600. Depending on the motherboard, you can take the FSB higher with the right components.
It's because the highest standard the chipset or processor is rated for by the manufacturer of the chipset is 1333, but the motherboard manufacturer guarantees that it will run at the higher speed anyways. It's technically a chipset/memory controller overclock, but they guarantee it will run regardless.
------------------------------Asus P6T deluxe
i7 965 @ 4.2GHz (200*21), 1.384V
12GB Corsair Dominator DDR3-1600 CAS 7
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