I was in the same position with Selhurst 6 - my lad wanted to see cat A games late in the season (Chelsea + Man C or L'pool) and we have been to 3 Cat B games already. Last week I phoned up and was able to get Chelsea tickets without getting a Cat B at the same time, no problem.
Perhaps you caught someone on a bad day!

But then everyone would get a 6, so they can go to the big games. Even if you want a box for a cat A game they insist you buy a box for a Cat B game too.its the same for everyone. Don't think it works,Ticket sales are far slower done this way and I managed to buy 2 tickets for Utd and Chelsea games on top of my ST. Had to buy tickets for WBA too but will sell these on half price or something.never thought this would be possible but the club putting them on sale straight after xmas means many are skint until they get paid.