Here is a better answer to my question. The salient information:
"Like its predecessor, the Ivy Bridge memory controller can work in symmetry mode (when the amount, clock rate and timings of memory modules in both channels coincide) as well as in compatibility mode which is referred to as Intel Flex Memory Technology. The latter’s point is in dividing the whole memory array into two parts, basing on the modules' specs: one with symmetric access mode and another with asymmetric single-channel mode. As a result, LGA1155 systems can be equipped with different memory modules without a catastrophic performance hit."
-- Bob Elkind