if you ask me, i would say it mainly b/c of 2 things!! Memory/FSB speed and L2 Cache!! remember Mhz ain't mean anything!!
i have 2 different system running seti, one is PIII 600 (coppermine core, 256k L2, 100MHz FSB), one is a PIII 450 (Katmai core, 512K L2, 100MHz FSB). guess which one crunch faster?! the Katmai 450 ave. 'bout 12hr flat, and the Coppermine 600 ave. 13-14hr/wu!