Provisional Matches?

Now first off, I AM NOT COMPLAINING. I genuinely don't care about my rank as long as I'm not B4 or B5.

This is what happened: I play 10 placement matches, like everyone. 6W 4L. I was pretty positive about getting to Silver, and then stop playing. Overall, really happy time. I played 2 games with my friends, where I had a mediocre KDA (somewhere around 6/5/8) and two where, in the first one, I got carried (3/6/15) and in the other, I made quick work of my enemies (12/2/3) with a near-penta kill. I got greedy so I died... For the other two, I played one ADC and one supp. The supp lane went OK ,and didn't win nor loose our lane. The final game, I played ADC and got a nice KDA of 10/0/9. So in total, 2 mediocre premade games with Yasuo, 1 bad game with Rammus, 2 good games with Jax and Trist and another mediocre game with I-dont-know which supp, most likely Taric.

And I was placed into.....Bronze 2,0 LP. 6 wins and I got Bronze. I was sure it was just a dice rool and I got 1, meanwhile my brother got 6 and with only 3 wins, got placed into S5. I was fine, since I don't care about my rank.

Then, 3 months later (NOW) I have a friend coming. He just started League, lv33 and he already played his provisionals. He got 3 wins, so I was sure he got B3 or B4. I was wrong. S4, is what he got. With 30% winrate. 3 divisions higher.

Now lets be clear. I don't need you to change my rank. I just wonder how this works and if wins even matter. What factor brings me below my run-of-the-mill friends in ranked terms. This is also meant for anyone with the same issue