This quote from pcpro.com should explain it...
It might seem logical to refer to these different modules by their internal speeds but, thanks to the double data rate that gives DDR its name, a 100MHz module can carry out a theoretical maximum of 200 million transfers per second, while the 200MHz module can carry out 400 million transfers per second. For this reason, 100MHz DDR is known as DDR-200, 133MHz modules are labelled DDR-266 and so forth.
Read more: What does RAM speed mean? | Analysis | Features | PC Pro http://www.pcpro.co.uk/features/230484/what-does-ram-speed-mean#ixzz37I1SJjdC