Question about Ranked

So I've played the 10 placement matches for ranked ,and I've won 9 out of 10.However after I finished my last game I got placed in Bronze 5,which is the worst rank.Note that this was my first time playing ranked and I didn't play in parties nor did I use boosting services or anything like that.Can anyone please explain to me why I got placed in Bronze,because I'm super sad right now,I thought I was a good player.Thank you in advance.