I suspect that the Ticket Office have drawn up various plans and scenarios and put them into some sort of business model. I.e. if we charge £x we can expect to sell y season tickets and the income will be z. However if we charge £y ST's and the income will possibly be >z. There will of course be a lower limit, where even though they will sell more the income will be less but I suspect they will go with the lowest possible cost, to sell the greatest amount of ST's, where their income stays generally in line with last season and make extra cash of match-day auxiliary sales.