Additionally some BD releases of Halloween and Mad Max have mono tracks that are actually downmixes of the surround track. The recent UHD release of Halloween sadly is one of them.
Then there’s also the fact that a lot of times, when original mixes are provided, they’re in much lower quality (say, 192kbps AC3) compared to the modern remix (often lossless). Older laserdisc releases’ uncompressed PCM audio will often be superior, even if it’s the same mix.
This is partly why I prefer the mono for Jaws on the 2005 DVD in spite of its minor imperfections over the mono track included on the BD.