Phenom II FSB overclocking help

Thank you for taking the time to check this out,
So I have had my phenom II x4 965 overclocked to 4.2ghz for quite some time, stable as can be. I did this by simply changing the multiplier from 17 to 21, and adding .025v to the CPU.

After some research I became interested in the potential performance benefits of an OC via the FSB and lower multipliers. I achieved 4.2ghz once more using a 210mhz FSB with a multiplier of 20, and it was stable under prime96.

Here's the interesting part, when overclocked to 4.2 via FSB it did 1M superPI in 24 seconds.. But before, when using only multiplier, it ran 1M superPI in 16 seconds!!

Why does it perform so much worse when I tweak the FSB??