I'm developing a game for the iPhone. I've decided that 30FPS is plenty so I've written some code that only allows the App to present the render buffer every 1/30 of a second. When I tried to verify this with Instruments I got varying information.

On an iPod Touch (2009 edition, 32G) it reports 30 FPS for Core Animation Frames Per Second.

On an iPhone 3G I get wildly varying results. And not just less than 30 FPS. I see >30 FPS on a regular basis. It actually seems to hang closer to 36-39.

To investigate this anomaly I added my own FPS to the app and update it once per second. I stays right at 29 FPS on both devices.

So, does anyone have any suggestions as to what might be going on? I expect Instruments to be accurate so it really concerns me that it appears inaccurate. It makes me think I have a bug somewhere, but I sure can't find it.