it's not so much how much it is off but that it's off at all. If the numbers don't add up then they don't add up. If there's any kind of difference then it has to be found and accounted for and it becomes a needle in a haystack search to account for the difference. Think about trying to find $0.05 spread across hundreds of thousands of transactions due to rounding issues.

If they could just pay the dollar and never have to worry about it again, sure. But the point is for them to have confidence that the math is unimpeachable and identical to whatever auditor or tax official would compute at every step of the way so you don't just have to guess at correctness with some waving of hands.