What you are both arguing doesn't match up with the facts. It was proved beyond reasonable doubt multiple times. The "proof", (which we now know was flawed) was that the system had shown that what they had sold didn't tally with what was sent to the Post Office, to refute that proof, the defence have to show another plausible explanation.
Yes, if they could have proved they also received what was stolen, that would have been a slam dunk but there are enough plausible reasons why they can't find the money. Maybe it was given to friends and family as cash, maybe it was used to gamble or to pay off some criminal.
It isn't much different than somebody saying, "you did it because we found your DNA". The Courts or Jury are inclined to believe it because "science" and if the defence are not on their game enough to show how "because DNA" is not always watertight, the defendent is seen as guilty beyond reasonbale doubt.