From IMUSA's site:
UNITED OVERCHARGE FOR READING TICKETS
IMUSA has close links with the nationally representative Football Supporters' Federation and thanks to these contacts it has come to light that fans buying tickets through the United ticket office have not been receiving the members' discount that Reading had allowed for and was expecting them to get.
The price for standard tickets for the FA Cup replay against United was set at £36 with Reading season ticket holders getting a £4 discount on this price.
Reading's standard practise is to pass on the same discount to away fans who are also season ticket holders. However, the United ticket office has been charging the full £36.
IMUSA contacted the Club and they have responded to this misunderstanding by agreeing to refund the £4 difference to those that have overpaid.