a question about ranked

If skill determines which league you're put into and bronze v is the lowest tier then why are diamond and platinum and gold players constantly showing up in my games? Silver I could understand, but why gold and above? Is this what happens when players restart their accounts? When players play together? Isn't that unfair to solo players? Why not just make it to where if you play with someone from a higher league then you have to play in their league but get rewarded more? Or maybe it's not that way and I'm just really bad at this game. I need to know.