Explain why this happens please

The game matches you by MMR. Sometimes, that will cause a discrepancy between actual player ranks because ranks are not based solely on MMR.

A player who just finished placements will almost always be put 3 or 4 tiers below where they should be. As the player completes more games, their MMR and rank will begin to fall in line. That is not to say that the MMR system is perfect. Frankly from all the games I've played, I only have a slight idea on how it measures performance. (Sometimes you lose MMR for carrying and winning). Perhaps one of your teammates is smurfing, and hence has a high MMR for his/her rank.