Hm. I've been asked that by people on my team and here's my best shot at answering via e-mail rather then in person:
The simplest answer is that because your device (router?) is not running at 100% all the time. In other words, rather then thinking of measuring from 100 and going downwards - breaking utilization into smaller and smaller components - it's the opposite.
Instead, the device, runs at some baseline idle CPU consumption - say, for example: 1 percent. As various processes (need to) consume more CPU time, they go up in their allocated share of CPU consumption.
A bad analogy: My boys youth soccer team with 18 teen-age players. When things are working well (assume a game) the 11 kids (and 7 bench players) are running around with little worry on my part because things are working well.
Occasionally, I might watch a player more carefully because they're about to take a shot or because they bumped into another player - hence, they consume more CPU time. But if you add up the "CPU" time I've spent on each player, it doesn't necessarily add up to 100. I've just spent a little time extra on one or two players.
Now if things are going badly, I've got kids being taken down from behind, a fight breaking out on the bench, and kids being tossed around on the field, I'll be pegged CPU-wise trying to handle and prioritize what to do with each of them. A measure of my CPU time then would be pegged at 99% as trying to think about each kid just 10% of the time would peg me at max.....
Fortunately, in my case I have assistants. :-)
(I make no warrants or guarantees about the accuracy of this answer. For a fully qualified answer, please contact the Cisco TAC!)
