The actual reasons for this are pretty simple.
Possibility 1: This 1 is true for 1 of the plats you played with. Not all higher elo players play normals enough to raise their normals MMR to their ranked MMR.
For instance, I rarely play normals because I don't like the inconsistency of the match making in it, ranked feels more balanced. I have 1,374 normal games and I've been playing since end of season 1 / beginning of season 2 area. Most of my normal games are from back when I was Silver/Gold. For the longest time when I did play a normals game once in a blue moon I'd get matched with players that I just stomped. Most the most part when I do want to play without really try harding I just go on my plat smurf, which is something I know a lot of my high elo friends do as well.
When team builder came out I did spam the shit out of that since it was my favorite queue by far and my MMR in team builder got up to around high diamond / low challenger. When team builder went away my normals MMR got a huge boost because they seemed to just throw our team builder games in with our normal stats. Now if I want to play normals alone I have a really long queue time and my games are extremely random. I'll get anything from an occasional silver to a challenger player in my games.
If you look 3 normal games back ,which was 8 days ago, the 3-2-7 game I had, my team was G5, G4, P2, D5 and myself, D4 while the enemy team was G5, P1, D5, D5 and D1. Unfair match making happens, but as you can see, it doesn't guarantee you will lose.
Possibility 2: Theres no limit to premades ranks in normals so it is possible for a person with low bronze MMR to duo with someone that is plat. The plat might only have Gold MMR due to the reason from possibility 1. Now add in them averaging their MMR with someone that is in low bronze and it will balance out to around high bronze / low silver.
If you look at my 5-7-13 teemo support game from 17 days ago my team was Level 30/D5 from last season, P5, D5 and 2 D4s vs Fresh Level 30,, G3, D4, D5 and Master. This was more than likely caused by premades. Because in that case I think the level 30 and master it made for some really wonky match making.
Tip if you want more fair match making in normals: Do full 5 man premades of people all around your rank. If you are B1 and have a full premade of B1s you will more than likely get matched with bronze and silvers.