This can happen because of the timestamp on the toll card. When you pass through the initial entrance point the timestamp marks the exact moment you passed. When you pass through the exit toll point the individual subtracts the current time from the time you entered the initial toll entrance. From this, he can easily determine the average speed that you were driving at. If the average speed is higher than the speed limit, it means that at some point between the two toll booths you were speeding and therefore they can give you a speeding ticket.
