The first mail will correspond directly to the query while the following others would show others as well. In spite of being one of the most used search engine in the world, Google has not being able to install itself anything other than that and it doesn't want to lose out on that USP. The upcoming social networking sites pose a serious threat to the search engine king and thus in order to protect itself and continue to number one, Google tries various methods to infuse innovation in its search machine.
Google Buzz which did not put up a good show failed largely due to the privacy problems but with this feature, privacy is not a big concern as once the user logs out then no information about his emails would feature besides search queries.