And more importantly different amount of propellant and different case lengths. All three main variants of the German 75mm tank guns used ammunition with different case lengths, 75×243mm for the L/24, 75×495mm for the L/43 and L/48, 75×640mm for the L/70.
True, but propellant amount is linked with barrel length - you need a longer barrel to benefit from more propellant, and more propellant to benefit from a longer barrel.