To avoid the problem of fare-cheats, i.e. people who pay the wrong fare, the system imposes a maximum deduction at the start and then a refund when the card is tapped again at the exit.
The system imposes a maximum deduction at the start. This was to avoid the problem of fare-cheats. The system imposes a maximum deduction at then a refund. This was to avoid the problem of fare-cheats. The card is tapped again at the exit. Cheats are i.e. people. These i.e. people pay the wrong fare.