Mint is based on Ubuntu, so don't get confused by that.
It sounds like you have installed more than one "desktop environment." No worries. Lots of people do this, on purpose or by accident.
You can choose which desktop environment you want each time you log in by choosing Sessions. Mint offers several different options, and I don't know which one you chose. Maybe Cinnamon? (their most popular these days)
You are still running Mint (based on Ubuntu) no matter which desktop environment you choose. The desktop environment is like a "skin" or "theme" for interacting with the operating system.