ah it's an interesting topic, generally this is called the Dunning-Kruger effect. 

What that means is that a novice can never know how little they know and therefore thinks they are better than they are. for example someone who knows very little english can't know how much english there is to learn and therefore might think the small amount they know is good! 

An expert however knows how big a topic is and despite knowing a lot feels that they still have much to learn, this causes them to underestimate their own abilities. 

If you look up the official definitions of these CEFR standards you won't be particularly more enlightened, they're very vague. It's even messier given that nobody's the same in all four skills, millions of English learners around the world have A2 speaking and listening and C2 reading and writing.

I don't take the little language level bars on italki seriously at all.

We had a similar discussion lately (https://www.italki.com/discussion/123965) - if you like to check out. 

The Dunning-Kruger effect is one reason but I also find it difficult to tell my level on this site, as there are no explanations. 

So what is level A1, B1, C1 etc.? CEFR? Or not? Because the names of the levels are the same - the (one/two word) explanations are not.

Also you can CHOOSE if it is the level that you are learning or the one, you already know, which is then not shown anywhere. Of course that does not explain people, who are at A2 and state to be B2.