UPDATE: I've figured out the deal with the Comcast situation where I'm receiving a /64 AND a /60 as replies.
Comcast always sends two advertisements - it's just that normally, you'd get a /60 from both servers (or a /64 from both if you didn't specify a prefix hint). Basically, this is their redundancy on the DHCP side of the house so that if server A fails, server B is also offering prefixes, and if A happens to be dead, then B's offer will still make it through and the customers won't know the difference.
It's just that people aren't usually scrutinizing this whenever things are working, so whenever you're getting the /64 from the "lease that won't die" - you're pulling out the sniffer and discovering a /64 from the main server and a /60 from the backup server that doesn't have you bound to anything at the moment.
I guess it's nice that they try to keep you using the same prefix, but if you switch from a /64 to a /60, it should stop trying to give you the same /64.
Also, I was able to match the DUID of the primary server with a content="\78\9A\BC\DE" rule entered on the terminal. I suppose that setting a different MAC address on the WAN interface would get you a fresh lease too, without having to break out the packet sniffer and the hexadecimal filtering rule.