small differences in brass shoulder geometry. they both can use similar projectiles and powder 'charges'
there is always a reason, and designs evolve. 223WYLDE seems to be the new normal.
when you begin to reload you will learn so much more, its a great hobby, do not reload to 'save money' … its more about learning to load the rounds that you like to shoot.
i reload 40S&W, 38Special/357, 380ACP, 300AAC and 6.5GRENDEL. i buy everything else because its just easier.
still its a great hobby, and can teach a lot about ballistics.
there is always a reason