If she jumped up she would just end up spoiling the guys party. You see Tidus, Auron, Braska, and Jecth all secretly had a plan to have a wild party and the only place to have it without getting caught was in the clouds. This made possible through use of anti-gravity belts they all secretly had. So Tidus made up some excuse involving the Fayth to escape Yuna to go to this party. Jumped off the ship and flew to the party, laughing uncontrobally the whole way there that Yuna actually fell for his story. Shuyin is so pissed off in X-2 because the guys never invited him to this party. JK i wanted to have some fun with this.