Here are the judge's instructions to the jury for one of the trials:
"There is no direct evidence of her taking any money [...] She adamantly denies stealing. There is no CCTV evidence. There are no fingerprints or marked bank notes or anything of that kind. There is no evidence of her accumulating cash anywhere else or spending large sums of money or paying off debts, no evidence about her bank accounts at all. Nothing incriminating was found when her home was searched." (The only evidence was a shortfall of cash compared to what the Post Office’s Horizon computer system said should have been in the branch.) "Do you accept the prosecution case that there is ample evidence before you to establish that Horizon is a tried and tested system in use at thousands of post offices for several years, fundamentally robust and reliable?"
My word against yours wouldn't be enough to meet the standard of "beyond a reasonable doubt", but the Post Office's word backed up by a computer system? It seems that was convincing enough for the jury. They gave a guilty verdict in the above case.
This sort of thing is why I've always hated the concept of a random jury from the general populace - In this example, I don't want Joe Bloggs the butcher's understanding of complex computer systems being the determination of whether I am criminally convicted, I want 12 people who have at least some experience programming.
I find it funny that juries are something that some people think is immutable while it's something that's unique to Anglo legal system. The first time I see jury in American media I'm immediately confused on why a random person can decide a court like that.
The problem was that it was more like a bank. Person gives you $9k cash, you add that to their account but it doesn't work so you push the button again. Now the computer thinks you should have $18k more cash in the till but there's only $9k there. There's no way for a bookkeeper to know that you didn't take that extra money.
"There is no direct evidence of her taking any money [...] She adamantly denies stealing. There is no CCTV evidence. There are no fingerprints or marked bank notes or anything of that kind. There is no evidence of her accumulating cash anywhere else or spending large sums of money or paying off debts, no evidence about her bank accounts at all. Nothing incriminating was found when her home was searched." (The only evidence was a shortfall of cash compared to what the Post Office’s Horizon computer system said should have been in the branch.) "Do you accept the prosecution case that there is ample evidence before you to establish that Horizon is a tried and tested system in use at thousands of post offices for several years, fundamentally robust and reliable?"
My word against yours wouldn't be enough to meet the standard of "beyond a reasonable doubt", but the Post Office's word backed up by a computer system? It seems that was convincing enough for the jury. They gave a guilty verdict in the above case.