Interesting, we only have 3 zones for both 320 (1-10, 11-20, 21-31] and 321 (1-10, 11-25, 26-39).
Entirely a matter for the operator and the weights engineering folks to determine how their loading systems are designed. The problem here appears to be that the relevant folks didn't recognise the mistakes made which ought to have been recognised at the time. Mistakes are part of the experience and we have to accept that they will occur - trapping mistakes before they become somewhere between embarrassment and catastrophe is the essence of what we do.