i.e. the 720x480 (NTSC) or 720x576 (PAL) pixels are all used for picture information if the source footage is 16:9, and the picture would appear vertically stretched (everyone is tall and skinny) on a 4:3 television, but no pixels are 'wasted' on black bars. But if the source is any wider than 16:9 then pixels have to be used for black bars.
Ananmorphic DVD *doesn't* mean that the DVD uses all the pixels for picture information regardless of the aspect ratio of the source footage.
If it did you would have different pixel shapes for 16:9, 2.35:1, 1.85:1, 2.22:1 etc. and the TV or DVD player would have to work it out, the people doing the encoding would have to have different workflows - it just wouldn't be practical.
With the move to fixed resolution panel displays, it wouldn't make any quality difference anyway, even if you used all 720x480 pixels to encode just the picture from a 2.35:1 source, you would still have to crush the picture down to 2:35.1 to display it on the 16:9 screen using the exact same pixels on the display as a standard anamorphic DVD with black bars encoded.
A non anamorphic (letterbox) DVD the 720x480 (NTSC) or 720x576 (PAL) pixels are *not* used for picture information if the source footage is 16:9, so black bars are encoded into the picture and everyone doesn't look stretched on a 4:3 television, but the amount of pixels used for the picture information is less.