It isn't, 686MHz is 1600 clock RAM, I forget what RAM clock is measured in and the conversion but yeah
Typically it goes something like this:
clock of RAM / x channels / x sticks per channel
So 2 sticks of 1333MHz RAM, each on it's own channel, would equate to about 667MHz per channel.
Of course, technologies like Cool'n'Quiet and manufacturers change clocks for stability or temperature management, so don't be surprised if it's not completely accurate.