I was just assuming that it's display only and not particularly tied to any change in the ranking algo. If a URL ranks for more than one query, the title rewrite changes for each case. So I was assuming that it's an attempt to show the user that "this result really is relevant for you" - something that happens AFTER the ranking is calculated.
So it's interesting that you see the changed titles right at the time your ranking increases. I'll keep an eye out for other clues, and would appreciate any other observations from members.