I did read the rest of your forum post, the most logical explination lies in how a half-bridge setup works.
if you run a trace route from you system you'll see a hop in there, the ip address of the modem, because in half-bridge mode it still working similar to a router.
Having said that, I stand by my answer, you're likely seeing connections that have expired on the dgl-4100 but have yet to expire on the modem itself.
Check out http://www.dslreports.com/forum/remark,14201975, it's quite insightful.