The long and the short of it is this: a processor's clock speed—measured in MHz and GHz—isn't a reliable indicator, all by itself, of performance. We've seen that time and time again, as Athlons at 1.2 and 1.4GHz have handed higher-clocked Pentium 4 processors their heads. On a platter. The Pentium 4's NetBurst microarchitecture simply does less work per clock cycle than the Athlon. That fact doesn't necessarily reflect poorly on the P4 or on the Athlon, it's just the way things are. An Athlon Thunderbird at 1.2GHz is roughly equivalent, performance-wise, to a 1.7GHz Pentium 4. A 1.4GHz Athlon runs neck-and-neck with a 2GHz P4.
Trouble is, the Pentium 4's super-high clock speeds tend to look mighty appealing on the features list of a brand-new PC. Given the choice between a 1.7GHz Intel and a 1.2GHz AMD at about the same price, the mythical Joe Sixpack is probably gonna opt for the 1.7GHz Intel. That uncomfortable fact threatens to become a marketing nightmare for AMD. Few companies would want to face the daunting prospect of explaining to Joe Sixpack why their 1.2GHz system is faster than the other guys' 1.7GHz box.
Apple tried it, but it didn't work especially well for them—and people already knew Macs were weird to begin with.
AMD's solution—check that, AMD's coping mechanism—is to try another spin at a well-worn tactic from the bad old days, when AMD and Cyrix CPUs played a sad second fiddle to Intel's: the Pentium Rating. Well, it's not exactly the Pentium Rating, but it is this: assigning a model number to a CPU based on relative performance rather than clock speed. (Source