If you look at your inventory of Payment Center licenses, you may come up with a discrepancy between the total used licenses and the actual number of licensed users. Take a look at the screenshot below for example:
It shows a total of two used licenses, but the actual number of licensed user is only one (that is displayed anyway).
This is most probably because the other license is used by the Site Guest User. The Site Guest User essentially allows your customers to pay online, and this user also needs a license.