The IRS works through several departments at once, so it is more than possible that the department in charge of sending letters worked slower than the department that actually reviewed your return. The letter states that you do not need to take action, and that once resolved, the refund will be mailed. Since you did receive the refund, it would seem that the IRS has completed their review. Remember that the IRS has the right to review or audit a taxpayer for years after the taxpayer files, so always keep copies of tax forms, and other pertinent documents and receipts.
