I dont understand matchmaking

I play pos 4 and 5 only and have fallen from Legend to low Archon over the past couple months. I admit I have fallen into the trap of trying too hard and raging since and during the big drop, but ive managed to claw my way back up from archon 1 to archon 3 the last couple weeks. Ive climbed again only using my best heroes which are lion/wd/undying pos 5 and lich/veng/jak pos 4. I try to itemise to suit the team and situation and try to play good support which is being unselfish, deep warding, de warding and generally trying to do what the team needs. Which leads me to my point - I dont understand matchmaking. since climbing again ive just now come off a 10 losses from 11 games streak. I just dont understand the matchmaking. lets assume this ridiculous system is geared to keep the majority at a 50% win rate. Statistically speaking, if I play exactly the same, should I at least have a 30-40% win rate minimum? reading this back to myself I think I need to uninstall this game ha