The system detects whether the source power is 30A or 50A, which it does by measuring the available voltage. If it shows 40A when on generator, it must have a special input wire that tells it the genset is running, cause there is no way to distinguish that otherwise. The energy management system in my coach just says "Generator".
40 amps @ 120v is 4800 watts. Maybe they thought that was easier to comprehend vs 45.8?
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL