CPU utilization numbers in ps and gr_top - what are they?

[David Hinds]

I'm curious about the CPU usage numbers displayed by ps and gr_top.  Are
they really what they claim to be?  Why do they always top out at 80%?
Why do they never seem to add up to a sensible number, and sometimes add
up to considerably more than 100% * the number of processors?

 -David Hinds
  dhinds at cb-iris.stanford.edu