How do you come to <100 when the ship didn't fire all its lasers during those scenes?
How can you be sure of Executer's compliment? Maybe the technical manuals were wrong. I certainly don't remember seeing a dozen laser points at any point on Executer in TESB or ROTJ. Maybe the lasers on Laviathan were ten times as powerful as those on Executer?
The point was that you can't really compare Executer to a 4000 year old ship of any kind.