I have not messed much with macros, but I would guess the reason you have to hit enter again after selecting your spell is that you can have one mob locked on, and then go to cast a spell, and change the target. I do it with AOE spells. single target first kill target, then AOE centered on a better positioned mob.

not sure if this helps explain why it happens for you or not, but thats my guess as to why they have done it.