I can't answer for certain, but you might find that the resolution size is due to hardware constraints, i.e. how the camera recieves the light on the cmos, during video not all the data gathered from the cmos is recoreded, it only uses parts (like a checker patern on the sensor) of the data (which is why you get those funny lines on stripes when you move the scene around, I forget what it's called) so you might find to evenly gather data of of the sensor you can only have that resolution... Once again, I don't actually know the answer, but it's a weird format seeings though 1080/720p are in 16:9 so its all I can guess that would make sense...
May I recomend just shooting in 720p... It means that you will be forsed to be organised and properly store your video and organise it... This will make editing and ordering of your videos after a few months allot easier and you can always downsize 720 to 480p (16:9) at a later point once you've found the footage you wish to keep... You can't upsize 480p (without obviously loosing quality)!!!