basically. pushing the timing and sizes makes it likely that some of your bits will fail to be built correctly. rather than dropping the speed and sizes to get reliability, you just throw an extra chip on to give you redundancy.

Take a look at the spec. The speeds are so high that they use some modem channel characterization features on the memory bus.

Linus was right about ECC being needed, with higher capacities and speeds and reduced feature size it's becoming a must.