Why is rot accepted on the stump but not on the top of a trunk?

During the optimization process at the log yard, where the trees are sorted for the respective sawn timber orders, a conveyor belt pulls the log to be optimized, stump first, past the divider. This means that the divider only sees any rot at the trunk cut, which it can then take into account accordingly. It cannot detect top rot. And so that the divider can rely on the fact that higher-quality sawn timber assortments can also be tolerated in the top area, the top must be healthy in any case.