I've just recalled something about the final Sephiroth Battle. It's one of a few battles that makes use of the 'Special' Pre-Emptive Strike setup. This means that the battle is considered as a Pre-Emp with two differences - 1) the enemy is facing forwards, and 2) all actor's ATB gauge starts at a random value.
All it basically means is that the first physical attack will hit for double damage, as it would in a Pre-Emptive Strike.
The only other battles to make use of the setup flag are the Wutai Pagoda Battles - Gorkii, Shake, Chekhov, Staniv and Godo