From what I understand, the MissingNo. glitch works by switching out your name for the old man's name, it stores your name in the thing that determines which pokemon you encounter in an area and so it bases which pokemon you encounter and what lvl they are on the letters in your name except for the first letter. Most people will have the last 2 slots blank, so there's usually always a chance of encountering a MissingNo. over lvl 100.
From what I remember, there's a few different variations of MissingNo. you can encounter through the glitch and you could also catch trainers too.