Outline numbering is able to get particularly confusing because of how 'customisable' it is. If you used one of the default outline numbering streams with styles then it can be quite simple but once you start customising you really do need to know what you are doing.
The best place to start in learning how this works is probably the late great Shauna Kelly's website
http://shaunakelly.com/word/numberin...numbering.html