It's a ratio.
if you divide them the opposite way ( 1:2 div -> 2/1, 4:5 div -> 5/4 ) and then multiply the result by FSB you'll get the RAM Frequency.
Example:
With an FSB Frequency of 333MHz, the:
1:2 Ratio will clock your RAM at DDR-1333 ( because: ( 2/1 ) * 333 * 2 = 1333 )
4:5 Ratio will clock your RAM at DDR-832 ( because: (5/4) * 333 * 2 = 832 )
etc