They do it because they want to make logistics simpler by using a single projectile type. They used to have lower caliber 23mm autocannon with less muzzle velocity. Used in Mi-24 like you said. Having more or less ammo is dependent on several factors. The Mi-24 was designed as a transport aircraft with heavy weapon equipment. Unlike the Apache which its meant to carry no passengers at all. So you have different weight allowances for gun ammo. It is not hard to design a gun with reduced propellant load either. The less pressure on the gun the easier it is to design.