Got it! If you're logged in and looking at a post, there's an author field
in the reply box that's autofilled (and not modifiable). When you search using the bar at the top of the page, the search function reads your login name in the author field of the reply box and feeds it to the search. I can also search for posts by you if I'm logged out: if I put "rick reynolds" into the author field
and hit preview, any search I make from the top of the page will find only your posts. Merely typing a name into the author field fails to exhibit this behavior.
I tried this on both Firefox and Safari (which are the only browsers I have at my fingertips at the moment) and the behaviors were identical.
Isaac