Then why only have one human bookkeeper? Surely two would be better, since you can compare their results. But then, perhaps you should hire three, so you can figure out which one is right.

Regardless of how correct they are, they assume liability: which is a metric that you do not improve with more bookkeepers.

But what if the third bookkeeper is malicious? You need at least four bookkeepers to achieve byzantine fault tolerance with f=1.