What's with the 720x480 size after encoding?

Thanks for SDMacuser's very detailed instructions in another thread (Can I make a DVD with multiple slideshows) - I saved the entire thing as a .pdf!

After I finished my movie, having had to deal with shots taken by four people with different cameras, settings, grainy or pixelated images, etc, etc, I decided to do a test just to see if different processes/settings would produce a better movie/image quality than others. I created a very short (3 images, all in different sizes and quality) movie and:

1. made one slideshow in Photo to Movie - specified size as 640x480/4:3 and exported it once as a Quicktime movie (expert settings, H.264, size 640x480, best quality). Second export for test purposes: directly to Toast (no options to customize settings available);

2. created a new project (DV, 4:3) in iMovie and imported the .mov file from Photo to Movie;

3. created a new project in iDVD and imported the iMovie project, settings 4:3 NTSC and "professional quality" and saved as disk image, and

4. created another project in iDVD ("best performance") and added a slideshow, dragging in the same 3 photos, saved as a disk image.

I then dragged each one into Toast and - finally (drum roll!) here is the question: every one of them showed up as "NTSC, *720 x 480*, 29.97 fps", etc.

Why would they all show as having been encoded at 720 x 480, which doesn't seem to be either 4:3 or 16:9, especially when I had specified 640x480 in Photo to Movie (if I specify a 720 width in Photo to Movie, the height automatically changes to 540) and specified 4:3 everywhere else? Two of the pictures were actually smaller than 640x480 and the third was 2048x1536.

I'm puzzled by this (odd?) size - is this something that iMovie and/or iDVD does or am I doing something to cause this?