In order to qualify as DVD-Video, the MPEG-2 must have specific pixel dimensions, bitrate, and other technical attributes, which differ if the player is NTSC or PAL. One peculiarity is that neither use square pixels -- so if the video does not looked squished or squashed on a (naive) computer-based player, it's unlikely to be strictly valid.
A newer DVD player might play "any old" MPEG file simply burned to disc, but for universality it must be genuine DVD-Video. (An older player might also have problems playing a burned DVD.)
If you use DVD authoring software, it will likely convert any video it accepts into a valid form, which requires re-encoding, taking time and reducing video quality.