I paid off the balance on my GM Flex Earnings Card from HSBC bank a couple weeks ago.  Today I double-checked the account on-line, and was surprised to see my balance was not $0.  It was $6.66.  I scratched my head for a bit, and then realized what happened.  When I paid the balance a couple weeks ago, it was the balance from my last statement, not the "pay-off" amount.  The pay-off amount is higher than the last statement balance, because interest charges are applied daily.  If I paid the statement balance the day it was posted, I would have been paid in full.  In actuality, I paid a few days after it posted, thus I owed additional interest for those days.