Michael (or anyone else), is there any visible reason why DDR3 1866 MHz RAM was sometimes on lower performance than the 1600 equivalent? On most tests it showed a relatively "linear" behaviour, but on others it left the queue with a jump.
I guess all of them were default and configured by SPD on chip/bar?
At a guess, the timings don't work out ideally with other clocks in the data path? If there is a 2:3 (or 5:4) ratio in there somewhere, it could make the performance a bit more weirdly sensitive to instruction timings and such.