Riot logic

So today my brother just finished his placement matches with a score of 5:5, which for some reason placed him in silver 4. For me, I finished my placement match with a score of 7:3, but I was somehow placed into bronze 3. Last season I was silver 4 while my brother was in silver 5 so my mmr after the season reset should be slightly higher than his, yet he got a free short cut out of bronze. Today was my silver 5 promo which I asked my brother to duo with me, but he just trolled first 2 games with garbage scores. P.S I am extremely tilted and tired at the moment so I don't give a damn about anything now, so feel free to say whatever you want