The story of 21/15/9
Coach has mentioned that these numbers came from a lot of different experiments with various number schemes, and it worked out best.
The decreasing nature refers to the “decay-rate” experienced by people who do them with a fixed weight. You do the first set of 21, and when you do the set of 15, your strength has “decayed” such that the 15 feel about as hard as the 21. When you do the final 9, it feels as hard as the 15..
Plus, all three numbers breakdown into 3s.
21 = 7,7,7
15 = 5,5,5
9 = 3,3,3