When you reach a certain level it becomes actually worthless to pursue small amounts. Compare to the story of the Midland Bank realising it cost them huge amounts in overtime having tellers go over every transaction to find a few missing pence per day in the cash desks; multitudes more than they were losing. They stopped.
I remember when working for the Alliance in 1980, spending pretty much an entire day chasing a discrepancy in an account of someone who had just died. It was a tiny amound (about 70p IIRC) that eventually I traced to a payment made in something like 1955, having spent several hours rummaging round a basement looking at old records. I thought at the time, my hours could have been better spent, glad there's some sort of common sense in the finance sector