Why does it matter?
You can reboot the router to clear them out.
I am guessing that they are remembered for some amount of time so that if they reconnect they will get same IP address.
Due to other reasons, I actually reset the router back to default settings and the inactive devices were still showing
The problem is not really a problem. Prior connections don't make a hill of beans on the functionality of your router. The current connections is all that matters.
and I don't keep accessing my router to look at my connections.
its like a Ron Popiel commercial, just set it and forget it. 😀