Would this be a HSBC or Orchard card, would it?
HSBC, and I'm sure there are others I'm not aware of, has an odd reporting schedule. For me anyway, they typically report sometime between the 5th and the 15th of the month usually by the time the statement cuts. However, they will post the balance usually that was on the last day of the month.
For example, I received the statement around the 20th of each month with a due date by the 15th of the following month. So, my statement was around March 20 and was due April 15. I knew if I wanted to have a $0 balance reporting, I had to payoff the card no later than March 31. If I wait until the due date, and my balance was, let's say $300 on March 31, then even if I paid it off on April 1, it will report as $300 on the CR.
And again, I'm sure there are other cards that report oddly like that. Which card do you have?