Well it turns out that when a game box says that it runs in 720p or 1080p it's not entirely true. Sure you can set it to that resolution but the actual game output resolution is rarely exactly that.
Check out two of the biggest releases of the season for example:
COD4 (360) 1024×600p 2xAA
COD4 (PS3) 1024×600p 2xAA
Uncharted (PS3) 960×1080 in 1080p
Neither Call of Duty 4 or Uncharted run in the actual resolutions.
The reason most games do this is to allow for more visual effects to be rendered to the screen without slowing the game down. Most won't ever notice or care but it's kind of interesting to find out the actual resolutions that games render in. Here are some more recent ones along with a huge list of older releases from the pixel counter at Beyond3D.