It has nothing to do with the 2 after DDR or the fact that it's dual channel. It's because it's double data rate (DDR) memory, and it can perform two transfers per clock cycle. The actual clock speed is 400MHz, but because it's DDR, and it can transfer two signals per clock cycle and it's advertised as being twice as much. I'm not sure if the fact that it's advertised as twice as much is a lie or just a technicality, but that's the way it is.