It's not less accurate. As commented above, the control knew they were being tested against the machine, so made sure to be super careful.
In everyday life the human is less careful, and the machine costs 1% of the human.
Because it's way cheaper. Not saying it's a good tradeoff, but that appears to be their pitch.
Presumably your current book keeper is not your slave, and you have to pay them...
...again: liability is the key issue. Cost savings a not exactly an isolated issue here.
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.
But what if the third bookkeeper is malicious? You need at least four bookkeepers to achieve byzantine fault tolerance with f=1.
does this assume a maximum of one malicious bookkeeper?