Stepping back a bit, it seems that prosecutors trusted the vendor's statement that the software was "reliable" in the sense that it provided accurate results every time, a subset of which is that it was tamper-proof.
WRONG
Not knowing all the details, it seems that no independent regression or red team testing was done on the software, apparently by choice of the PostOffice and prosecutors and by the inability of defendants to gain the resources or clout to do such proof of unreliability, and also apparently due to the closed black-box nature of the software.
The BBC title is "NI Post Office ex-investigator apologises to Horizon victims". This submission title makes it sound like the opposite despite the quotes
100%?! No system is 100% reliable in a timeframe.
Maybe the system was `eventually` 100% reliable.
The UK just doesn't have a functioning judicial system.
Not only were hundreds of people falsely accused in these cases, they couldn't adequately defend themselves. Now that wrongdoing has been exposed -- the post office knew about this faulty system for years, nobody is charged or going to prison for years of damages due to false prosecutions.