Im not sure if this is the case with the Pentium M and P4 but the measurement of ghz is flawed.
For example, the 2.8 ghz AMD FX-59 can tear up and Intel 3.8 ghz because the AMD uses shorter pipelines than the intel.
so, if you had an object going 28 feet per second, but only had to travel 75 feet it would finish faster than an object going 38 fps a distance of 120. Catch my drift?
I believe M's are smaller than P4s so this is a reason for speed increase.