video - Why don't some subtitles match with the movie?

First, the video you download can be edited. Those stupid advertisements can be cut out, or "Downloaded from *" added.
Next, after many container conversions (HD->mkv->streaming->mkv again)the alignment of tracks can shift due to bugs in software.
Finally, some older subtitles where keyed not by time, but by frame number, and modern codecs have variable frame rate.

I use VLC player on my PC, and it allows me to adjust timing of all tracks.