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.

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.