Any idea?

I have noticed it does that on ALL levels (no matter whether it was classic or otherwise when you started playing the level). Once you fail, it labels it super hard and offers you the option to watch adds to get more moves (twice times two moves) and get a color bomb (for the third commercial). I simply assumed it was due to the fact that you failed to pass the level on the original try.

They can be -- a lot depends on luck and what happens in the first couple of moves. Between the starting bonus from watching an ad, the bonus from the character start, any sweet streak bonus special candies and bonus timed boosters from dach's or similar, there have been super hard boards that clear nearly in a single move (like when you combine a color bomb with a coloring candy) or boards that become nearly impossible if not impossible (like when the initial move that would clear a necessary part of the board doesn't get to be played because one of the above changed colors or made the move unplayable).