You get unknown skill once Dota Buff can not determine your skill bracket which they receive from steam. That is when, they make it unknown skill then.
Its your almost new ID bro. When your ID is new, you can jump even a 1000 MMR in a single match depending upon your performance. Because till then, your skill is not finally established by Dota 2. You must have played very well recently, therefore, Dota 2 moved you to next skill bracket.