Because not everyone can get their 2000mhz ddr3 to perform at that speed. Also according to a review Anand did a while back. 1333mhz and 1600mhz ddr3 at cas 6 and 7 beat out faster 2000mhz at cas 8. Seeing as the memory controller is on the cpu, pure memory speed probably doesn't factor as highly as it did in the past.
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