It’s a simple progression. The logic is that a barbell is heavier than a kettlebell, which is heavier than a med. ball, which is heavier than no additional load at all.
It’s not a rigid 2 year lattice written in stone. Some kids take longer, some less and there’s value for the stronger athletes to incorporate the lighter stuff as well. Really nothing earth shattering happening here. I just wanted to say that kettlebells are a nice introduction to weights. It avoids lifting too heavily too early. Especially boys are tempted to outlift each other. And they provide variety, which the girls in my group in particular appreciate.