pixel size question

Why is it that when I open a movie in Quicktime and Apple+I it, it shows two pixel aspect ratios.

There's one normal and one in parenthesis.

Ex)

720 x 480 (853 x 480)

Why is that and which is the true ratio?

That indicates an anamorphic clip. DV NTSC is only 720x480. It can be 4:3 using rectangular pixels, or anamorphically squeezed.

An educated guess: the first one is the true resolution of the media. The one in parentheses are the pixel dimensions it's displaying it with. In this case it's anamorphic NTSC, and it's stretching those 720 pixels over 853 screen pixels (and blurring it in the process).