In Test levels, you try to pass the test. Sometimes that can be hard.
In Single Player levels, you fight villans and bosses! That can be hard.
But in Battle, CTF, and Race, you're just fighting other players. It can't do much when you beat them! In Test levels, you might get in a hall of fame. In Single Player levels, you get to say you did them, and you can also see what's next in the levels!

Single player levels may have that like tests, but you can't host them. Unless you have XJJ2.