The performance of a computer is measured in Instructions per second
and NOT "cycles per second"..
Cycles per second is what u are used to - the GHz value.
In each "cycle" different CPUs do different amount of work.
Instructions / second = [instructions/cycle] x [cycles / second]
AMDs run at a "lower" [cycles/second]. But they have "higher" [instructions/cycle].. This is why the "GHz" value doesn't tell the full story
Just picture 2 people going to work everyday - one person might do MORE work than the other person each day, even if he goes LESS days to work
.. What matters is the total work output - not how many days the person goes to work