Explain to me the logic of how MMR works

I've been playing this game for like 6 years. I have 3 ranked accounts right now. My newer account I just got to 30 and played my first 10. When I checked lolnexus most people in game were silver 2 - gold 4 most of the time. THESE GAMES WERE EASIER. I went 4-6. I was placed in silver 4. HOW DOES THAT WORK? I'm placed 3-4 ranks below who I was playing against on a fresh account? That doesn't make any sense. I played two more games after that playing my best and all my teammates feed. ARE YOU SERIOUS. Riot you better see this post and promote me to at least silver 2! Get rito