Why do certain tasks in the profiler vary wildly in duration?

Hi!

I'm trying to track down random spikes in random functions on my iPhone 4. ( You can see my more detailed forum post here )

For kicks I started up a completely empty, brand new project on my iPhone 4 and profiled it. I see the obvious spikes in Device.Present where it's waiting to VSync, but I also see wild swings in the timing of other functions- sometimes camera.render takes 0.5ms and sometimes it takes 3.5 ms ( with most of the CPU time spent doing the Clear ). What's going on here? Why aren't the timings consistent? This is a freshly restarted iPhone 4, FWIW.

Thanks, Josh