Why are there promotion series within Leagues?

You're telling me that I need to win 70% of my games in order to advance from Gold 4 to Gold 3? Not only that, but there is a system within the match making that purposely matches you with players who are on a lose streak and stacking the enemy team with players on a win streak. Don't believe me? Every promotion that I get into, I check the enemy profile and my team profile and my team always has the losing streaks while the enemy team always has the winning streaks. I can understand there are promotions from Gold 1 to Plat 5. But to have them within leagues is retarded.