How is optimal CPU frequency computed in Vista?

Why does the CPU run at less than 100% of maximum frequency most of the
time, even while CPU usage never drops to 0%? Vista (RC2) is showing the
graph of my CPU usage and frequency, and CPU usage is averaging about 40%
while frequency is averaging about 85% of maximum. During the times when the
CPU has anything to do at all, shouldn't it be running at 100% frequency,
and then drop down to 0% (or whatever minimum frequency is necessary for
internal memory refresh, or whatever minimum frequency the processor
supports) until an interrupt is received to trigger more computational
chores? Since this is a desktop machine, and performance is more important
than saving power, why is the CPU ever running at any frequency between
minimum and maximum?