I had a similar issue last season, for the Man Utd game (I'd always bought tickets for me and my son on the one customer number). They took one look at my purchase history and were happy to sell me two tickets over the phone as it was obvious I wasn't "trying it on". Maybe it depends who you get through to on the ticket line as to what level of discretion they use. Or maybe they've changed the system so it doesn't allow for discretion anymore, which would be a shame and a step backwards in terms of customer service. Maybe phone a few times in the hope you might get through to someone who's able/willing to 'bend the system'....?
I've now set up an account for my son and always buy a ticket on each one.