Many people won't play their main champs or even their main roles in normals though, so while I might be a diamond level janna support player, I'm probably not even a silver level lee sin jg, and probably a goldish adc player.
People who don't play normals very often aren't going to throw off very many matches, and the ones who do are going to end up at an appropriate normals mmr for the level of play they're exhibiting there, and it's very possible that that level of play is quite different than their rank would indicate as they're playing with new champs/builds/roles.