The 20% could easily be missed if it ended up looking like a ticket wasn't sold at all (ie. just 20% fewer tickets got sold).

Many railways don't keep track of the paper that tickets are sold on, because the same paper is used for printing receipts, seat reservations, free/discounted tickets etc.

Many railways also have a double digit percentage of fare evasion. Also, many railways operate partially/mostly on government funds not ticket revenue, so a 20% loss in ticket revenue wasn't a 20% drop in total revenue.

But they had a record that said how much he stole. So they knew how much they were supposed to earn vs what they did earn.

But surely they would have a counter of how many tickets were sold in a machine?

I do wonder if the repairman had the ability to reset/twiddle with that counter...

It literally calls this out in the article folks. Leadership attributed it to a software bug and didn't look into it further.

Given the Post Office / Horizon scandal, I'd actually be reassured the default assumption was the software was wrong.

Further reading at https://en.wikipedia.org/wiki/British_Post_Office_scandal , the TLDR is the biggest miscarriage of justice in UK history because people insisted the software was right.

About to be the subject of a UK TV series this month.

Well that has the potential to be incredibly depressing. It will be super interesting to see how they portray the developers and if they call out fujitsu

I really hope so. The authorities haven't even compensated most of the victims yet, still stalling. Hopefully the series will shame them into doing the right thing. It was disgusting what they did to these poor people, mostly just small mom and pop businesses who couldn't fight back.