Well, this might be the department of "What is the question?". Aurora is the build that's supposed to be updated even faster than Beta - it's supposed to be the more stable cousin of Nightlies.
Thanks for waking me up! I see that I deployed my customary level of ineptitude when I posted my question.
I meant that my copy of Aurora is at 15.0a2, and reports itself as such. Despite that I get daily prompts to update to 15.0a2.
So, even with a fast updating cycle, I had expect to see some change in the version ID. Perhaps I'm wrong about that. Next time I should check the file sizes etc. pre/post updating.