It wasn't though. If the post office enforcers had taken even a cursory look at the transactions around the 'thefts', they would have noticed obvious errors. One of the bugs basically just duplicated a close-of-day transaction, sometimes many times. This would obviously have looked like an error, it would be a stupid way to commit fraud. It was obvious that the Post Office just preferred to extort money out of the postmasters as opposed to actually work out what was going on (as evidenced by the bonuses for successful payments or convictions)