it's there because of Mint is built on Ubuntu (like Ubuntu is built on Debian)
We have not had the time to change the update manager so it does not show that. I intend to look into that if/when I get some time to do it.
There was a sticky in the forum advising against it but I removed it after Cassandra had been out for about a month.
The upgrade procedure from Bianca to Cassandra is described here
http://www.linuxmint.com/wiki/index.php ... _Cassandra
Upgrade from Cassandra to Celena is to be done from the software portal (by a .mint file) and so much