[SOLVED] How is the inbox sorted?

I'm here staring at my inbox for the third time trying to figure out how the hell the sorting in it works. It isn't alphabetically by subject or user, it isn't the recency of the first message (nor is it the recency of the latest message), and it isn't the number of messages. Testing it now, it also isn't the most recent you've opened, so I have no idea how this works or how to look for anything, not that it's big enough to matter much yet.

Am I missing something obvious? I feel so lost.