These categories have grown up gradually. It may be that we could do with fewer.
Developing and customizing SMF are clearly related. Theming is, arguably, a sub-category, because, of course, it does involve SMF code.
As doc lead, of course, I am willing to take a lot of guidance from dev team, customization team, and some knowledgeable non-team members on how to categorize this information. My aim has to be to make it easier to use the info, and also to figure out where/how to add additional info.