What's with the 720x480 size after encoding?

Question

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?

Answer 1

Generally speaking it's:
  720x480 @ 29.97 fps for NTSC (or 23.976 for NTSC Film)
  720x576 @ 25 fps for PAL
Anamorphic video formats (e.g 16:9 aspect) is accomplished either by (rarely) scaling the image down and including black pixels at the top and bottom of the image, or (usually) by the setting a flag which tells the DVD player to stretch the pixels horizontally, scaling the image and automatically adding black at top and bottom if needed.
   !http://img228.imageshack.us/img228/5186/scottdartve1.png!