Host Clock x multiplier =
CPU speed
If
CPU is multiplier locked it could be that the multiplier is too high to overclock the FSB too far, dropping the multiplier may increase the ability to increase the FSB, eg. a processor that can do 1.67GHz
12.5 x 133 = c.1667MHz
10 x 166 = c.1667MHz
Obviously the processor that is unlocked and can do the lower multiplier in this case will outperform the locked processor since its memory speed is higher due to the higher host clock speed.