Popular topic for the moment. It seems that Debian may be using portions of the Ubuntu Linux operating system and repackaging (rebranding) it for use in their operating system. Specifically the Ubuntu Software Centre but with many of the Ubuntu-specific features disabled, as shown below:
There's an official bug report created documenting this issue:
There is a "Notify me of a new Ubuntu version" setting in the dialog that appears by clicking on Configuration, then Repositories, then clicking on the Updates tab. What is it doing in Debian?