A Shocking Rashi

A topic that has been troubling me for some time. The gemara [Rosh Hashana 17] says that whenever we say the 13 middos our tfillos will be answered. Rashi, in a SHOCKING TURN OF EVENTS completely explains the gemara against the simple explanation and says that this means only during fast days. What compelled Rashi to make this comment? The gemara says that it always works and Rashi says that it doesn't. Tonight I sat down after evening services at the local Orthodox synagogue and went through everything I could find on the topic.

There isn't much... Here are my notes with sources for those who want to research further.

A methodological note: Reading Rashi carefully yields great dividends! [I especially recommend the sfarim of Rav Homnick Shlita - most of them called "Marbeh" something, who takes learning Rashi to a whole new level.]