The only relevant Meta question I found was Moderator re-tag requests: [characters] and [character-naming] The question is about a different but related problem, though character-development is mentioned in an answer.
Sometimes a question can be about other aspects of the character than character-development. For example, my own Writing diversity is about having a diverse cast of characters. It's not about how the characters grow and change - it's about having them there in the first place. Another question that fits one tag, but not the other: Writing witty/funny characters with deadpan humour. On the other hand, I can't think of a question that would go under character-development but not also fit under character.
This is not the only case we have of one tag being sort of a sub-tag for another. Consider fiction or creative-writing: anything that discusses characters would probably also fit under those two, right?
My own tendency is to apply to a question the tags that are a bit broad (e.g both character and character-development) but not the tags that are very broad (e.g fiction) if there's a more specific tag to be found. My logic is that if someone is looking for information on an aspect of character development, he might look under "character" - it makes sense. So it's useful to apply both. But looking for the same information under "fiction" would lend him more irrelevant information than relevant information, so there's no sense in applying the tag - one wouldn't be looking for this question there.