Not sure. I think the bottom's probably the hard drive bit. Routers aren't actually that big - just a circuit board, whereas hard drives are quite thick (sometimes an inch thick) so it'd make sense for the TV & Wi-Fi (core package) to be in one box, with the PVR (optional extra) in another that just bolts on when or if you subscribe.
Also, when the guy came to install my HC about a year ago he said they were working on a wireless box to plug into the phone, which sends the telly signal as well as broadband signal to the STB - saves engineers trailing wires around and makes home installs much more efficient (so they can lay off some installers and still get the same amount of work done). Maybe that's why the Wi-Fi's now built in.
Thinking about it, it makes sense. Even with 8meg broadband there's still lots of room to spare in a router's 54mbps capability.