Payment

My guess is that the system does not distinguish between credit and debits cards when it comes to providing error messages.

Have you called your financial institution about this issue? I think when you enter a card number Apple just contacts the financial institution and asks, "Is this card good?"

I don't think Apple's system is distinguishing it as one or the other, it is just seeing it as something it can charge to get the money for the item you bought. If there's an error message associated with this it might help if we knew what it was exactly.