Because he possesses the Primary Divine Skeletal Manipulation, he doesn't only rely on the Skeleton Cannon's own ability—the Skeleton Giant Cannoneer's Life Dice only determines how many skeleton cannonballs the cannon can fire each day, and his skill level also slightly determines the probability of hitting targets.
The power was fixed when the Skeleton Cannon was created, as its strength is determined by the amount of soul energy drawn by its magic array unit over time.
It's different in the hands of the Giant Bone King; he can not only use his Life Dice as extra damage, just like the Super Death Stalker creating a Skeleton Cannon in his own body, but the condensation speed and quantity of skeleton cannonballs are incomparable.
The reason is simple: he can use Primary Divine Skeletal Manipulation to control the bones and assist the Skeleton Cannon in attack.
