The Horizon system failed to keep accurate records. It was a kind of distributed database, with individual users entering data on their terminals and occasionally connecting (potentially via dialup, iirc) to a central system to sync their records. But it was implemented badly, and records could and did get lost or modified. Records could be edited at any time by any of several parties, including the postmasters, but then when they were synced those edits could be silently lost. The resulting ledger does not and can not accurately reflect reality, thus the lost money and the convictions.

Assuming that an itemized breakdown was requested, this is a possible answer why it can't be provided. How can a prosecution continue following such answer is a different question.

The prosecution would happily provide the itemized transactions, because the missing transactions are exactly what they are prosecuting. They know there were N pieces of mail that day, but payment was recorded only for N − 2 or whatever.

The lies and coverup from the Postal service and Fujitsu were to there to hide the flaws in the system from the courts and the defendants. Even the prosecutors have said that they were completely in the dark. It’s only now that the independent reports are out that everyone knows the details.

Unless you know about the bugs in the system, the itemized transaction list makes it look exactly like the cashiers were pocketing cash and not actually ringing up the sales. It’s a classic scam that any competent retail manager or owner knows to guard against. The only difference is that these cashiers nominally own the stores they run. If they pocketed the cash from the sale of a candy bar then there would be no crime; the cash was theirs to begin with. But since they are also working for a third party (the post office), the possibility of theft is a real one.

If the money was eventually handed over to the PO, would it not be reflected in a ledger?

I feel like I am repeating myself.

You would assume so, but the whole problem was that the Horizon computer system would lose transactions. It might lose the transaction where the customer paid by credit/debit, and the post office received the money directly. Or it might lose the transaction that recorded the check the postmaster sent to the post office to reconcile accounts. Again, the Horizon system never had a reliable record of what transactions had actually happened. Once there were discrepancies, it was always easy for it to look like the postmaster had made a mistake or committed theft.

I didn't realize that Horizon was used to record "the check the postmaster sent to the post office to reconcile accounts".

Yea, it was their entire bookkeeping system. Obviously nobody else has ever had so complex a bookkeeping situation as The Post Office, so they had to get it custom built. I bet Fujitsu made a mint.

> Records could be edited at any time by any of several parties, …

I took an “Accounting for managers” course back in the nineties which included double entry book-keeping - we were taught to never erase a mistake but create additional entries to correct any error. You’d think something like Horizon would adhere to agreed standards of record keeping and have a signed and immutable event log at the very least.

Yep. I’m not an accountant or even a manager, but even I know that. Apparently the Post Office testified many times that the transaction record _was_ immutable, even though Fujitsu knew that records could be edited or even lost at any time. And it appears that their own customer service employees would edit transactions all the time, to correct perceived mistakes. And it would appear that this might even make the problem worse, as the Horizon system could fail to correctly merge those changes down the line. It could silently revert the edits or silently drop entire transactions as if they had never happened.