Most films on NTSC DVD are progressively encoded, no interlacing, to get the 23.976fps up to 29.97fps of NTSC the mpeg2 stream contains flags telling the player to repeat certain fields (3:2 pulldown), but if you play the video on your PC which is has a truly progressive output (your CRT or LCD monitor) the software player ignores the flags in the mpeg2 stream and plays it at 23.976fps.