If you have purchased a PMP 450 Lite version in the past, you may have upgraded it to register more than 10 SMs.
An easy way to tell if the AP is a Lite or a Full version is on the Device Information Tab on the home page of the AP GUI.
A Lite version will appear like this (Note the "L" in the Software Version:
After upgrade (or on any other Full version), it looks like this: