With DHCP you get a "lease" on an IP for a certain amount of time. This amount of time can be specified in the router settings and can vary from an hour to weeks/months/years/whatever you want.
There are one of two things happening here:
1) Computers were connected to your network at some point previously and their lease has not yet expired, thus they are still showing up in your client list
2) Computers currently are connected to your network, which they can still do if you have not changed your network authentication key, even though you have MAC address filtering enabled
Check your DHCP lease time to see if #1 is plausible. I think it's on the basic settings page. Also, if you're really worried, change your network authentication key, use WPA2, and use MAC filtering to only allow those listed (which it sounds like you're already doing).