From: United Kingdom
One issue may be that in recent CPU models, Intel were rather slack on some of the more basic elements, delegating some complex Integer operations to the slower floating point ALU. One version apparently lacked integer multiply hardware, but could stream adds very quickly, laeading to a code-hack that used multiple adds up to a certain level of multiply.
Presumably, the games tend to use simpler operations that Intel forgot about (eg. they'd recommend using SSE/2/3, or splitting it up for hyperthreading), while it's well known that AMD's implementation of SSE is not up to the standard of Intel's - for AMD optimization, 3DNow should be used even on the ones with more support for SSE.