Comment Fees are probably higher for these transactions. (Score 1) 215
I can't say for sure for these one time CC #'s, but the difference in fees for "regular credit card" vs "gift card" can be up to 100%. That means 2.5% in fees to the merchant for regular, 5% for gift card. This is to cover the perceived change of fraud. Transactions where the card is swiped vs. ones where the number is punch in manually will have different fees as well.
Of course, the merchant can just decide to deny any card that causes them higher fees. That's probably what Blizzard was doing to you.