Firstly, PAL video runs at 25fps, NTSC runs at 30fps.
Wouldn't movies on NTSC then be shorter than on PAL? But it's the other way round.
Goldfinger (PAL-DVD): 105:26
Goldfinger (NTSC-DVD): 109:52
So how can the NTSC-version be longer when they show 6 frames more per second?
I found the answer:
Film (in the cinema) has 24 frames per second
PAL runs at 25 fps
NTSC runs at 29,97 fps
"Framerate conversion
There is a large difference in framerate between film, which runs at 24.0 frames per second, and the NTSC standard, which runs at approximately 29.97 frames per second.
Unlike the two other video formats, PAL and SECAM, this difference cannot be overcome by a simple speed-up.
A complex process called "3:2 pulldown" is used. One film frame is transmitted for three video fields (1.5 video frame times), and the next frame is transmitted for two video fields (1 video frame time). Two 24 frame/s film frames are therefore transmitted in five 60 Hz video fields, for an average of 2.5 video fields per film frame. The average frame rate is thus 60 / 2.5 = 24 frame/s, so the average film speed is exactly what it should be. There are drawbacks, however. Still-framing on playback can display a video frame with fields from two different film frames, so any motion between the frames will appear as a rapid back-and-forth flicker. There can also be noticeable jitter/"stutter" during slow camera pans."
So the NTSC-DVD got almost 30 fps, but has the same runtime as the film in the theatre. If they would speed up the movie to 30 fps, it would be much shorter than the PAL-version.