There were many things Aether couldn't understand, especially his Survival rate...
There were times he thought it was related to his current survival situation, but he quickly dismissed that notion. After all, most of the time he faced numerous life-threatening situations.
So, his survival rate never meant his current situation, rather it was related to the 'Survive until the Beginning' mission. This meant his survival was purely based on the 'Beginning' that the mission talked about.
Even more perplexing was when he encountered the Empress. She posed a significant threat to his life and drained his survival rate, yet when the Archpriestess came into action, the Empress's presence seemed to raise his survival rate, as if he needed both of them rather than just one.