Post Office told the post masters that they were short, so many post masters made up the shortfall from their own pocket, expecting the books to eventually balance and to get repaid.
When people were unable to continue making up that shortfall this was seen as further evidence of their criminality: "they've spent the money", "they've hidden the money", and not "they never had the money".
It’s insane they stopped pulling the thread and the defence didn’t push that, even if you spent it there would be evidence. I would have been highly skeptical that all of it just disappeared Into thin air across the entire group....nuts
That's financial crimes 101