It is because fat man is a wizard, Cait uses magic to reload her abilities (Where do you think she keeps all of those nets? She teleports them to her gun then uses them.) GP with his Q to ignite the gunpowder (And reloads by teleporting a bullet into his gun before firing) and Lamb uses magic to propel herself when she jumps by jumping slightly, using magic to fall down at the end of the frame, and then convert the momentum of her hitting the ground back into her hooves while jumping at the same time to get a much faster leap and give time to fire the arrows midair.
Simple really.