We don't have all the facts or history of the debt so we can only guess. Consider that the original debt on the card was accruing interest each month when you went into default. Then when it went into collection, the cost to get a judgment, legal fees, and continuing interest continued to be added. So it is possible that the original debt of $6,500 grew to $18,833. They took your tax refund dropping the judgment to $13,333 where it still accrues post-judgment interest.
If you never received any service on the collection case, never received notice of a court hearing, and a default judgment was entered against you, you should talk to an attorney. When the case was filed and when a default was entered may be affected by the statute of limitations. But as I said, we don't have that information.

