How does normals matchmaking work?

https://s32.postimg.org/du7txldut/image.jpg so I'm Bronze 3 and played a game with 2 silvers a gold and a Diamond vs 3 silvers a gold and a bronze

how does the normal matchmaking work... are there separate mmr's(like norms mmr and ranked mmr) or is tier matching just really lenient or something

granted i like the mix of skills in normal's matches... it feels good being a bronze and beating a silver with a insane counter pick(the diamond kha had a lot to do with it but shhhhh x_x)