The placement determination takes into consideration your placement games W/L ratio and Rank last season, when determining your initial Ranking. Most likely saw the discrepancy between your MMR and Ranking, adjusted you down a Rank. The system dose this because it is designed to not to put you way above your skill level when initally Ranking you. If you keep playing and consistently winning (55%+ winrate), you will climb up to gold or even higher!
Ultimately your initial Placements don't really matter in the long run. The 30+ games right after those placements are WAAAAAAAAY more important. I also wouldn't put too much stock in a Rank until someone has over 200 games played.