I've just started offering bounties on my questions, and to me the first two possible reasons for offering a bounty seem like the same thing:
- Authoritative reference needed: Looking for an answer drawing from credible and/or official sources.
- Canonical answer required: The question is widely applicable to a large audience. A detailed canonical answer is required to address all the concerns.
Maybe the difference is more obvious to others, but to me it seems like these two statements mean more-or-less the same thing. So this is my very humble suggestion to clarify the wording or merge these two options.
If the only use of the bounty reason is that it's displayed in the bounty description box on the question, then the overlap might not matter in practice. But if the data is used elsewhere then maybe it would be nice to clarify.