This is the fault of the memory companies; there are over a hundred products being sold, supposedly made for i7/i5 platforms, in at least six speeds that are not supported by the processors! Some of them can eventually be 'tweaked' to make them work; some cannot on particular motherboards; they don't care - they want the money! They can charge over a 200% premium for higher speeds that will either make no appreciable difference in actual system speed, or simply not ever work right. The computer press doesn't care either - they are dependent on manufacturer's good will for samples and, more importantly, advertising!
Caveat emptor! "He buys at his own risk"