Pal & NTSC subtitles

[atropos]

Why is it that subtitles in re-authored PAL movies always stay in sync throughout their duration whilst NTSC movies start off in-sync but at the end of a movie the sync difference is several seconds.

atropos

[screw]

It is because of "drop frame" functionality used for NTSC DVD-s. If "drop frame" functionality is used, frames-to-time differs from "non drop frame" case.
Subtitles are always time related, so if they are going out of sync at the end of NTSC title, it means incorrect use of "drop frame" feature when authoring.