Every relation with a bank assigns you with an account number, savings account is a simple example. your account number is only number in this case. In a credit card however, the bank creates a credit card account at the backend. this credit card account number is different than that of you credit card number. when you upgrade/reissue you card, the card is changed but it is mapped to the same credit card account number. This has been my experience with icici, hdfc and amex.