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.