I noticed that when I moved over to the new, shiny Mac Pro that I hadn’t brought over all my digital photos. I’m glad I realized this before whacking the hard drive in the old Windows machine… Anyway, I brought over 2Gb of photos and started importing them into iPhoto. Shortly after it started, I noticed my iconified activity monitor showing activity on all four cores of the CPUs, so I opened it and saw a very large number for CPU percentage for iPhoto. I assume it’s some sort of sum of cpu utilization across the cores, but it looks pretty funny, however they compute it.
Joey Gibson's Blog 
This is common on all versions of Unix: if you have more than 1 CPU (or core), you have multiples of 100% of available CPU.
My guess is you have a quad-core system, so you could potentially see 400% CPU utilization.
Cool, huh? 🙂
It’s two dual-core processors, so there are four cores. I’ve never had a multi-core system before, so I’ve never seen this before (though I’ve worked on Unix systems for 16 years).