Comment 3 for bug 237662

Well, the proximate reason why it's still offering you updates is that aptitude didn't upgrade some things (the bits it said had been "kept back")--aptitude can be a bit picky about some upgrades. There will be some way of forcing it to install these using aptitude or apt-get, if you particularly need them.

The ultimate reason for the problem is presumably that update-manager is hanging somewhere--it may be failing to connect to the server, for example (is there anything odd about your internet connection, such as a proxy server?). I've marked the bug as affecting update-manager.