Flaw With Ranked Placements :(

I was silver 5 last season (during season 5) and I was very exited to finally get to do my placement matches. I worked very hard and got a very good score on my placements of 7 wins and 3 losses. When I was assigned my rank however I was surprised. I was assigned bronze 5. After going 7 and 3 in placements and starting in a whole division up. I feel like riot needs to improve the ranking system so this kind of mistake should never happen. I probably will play a whole lot less league of legends now because of this unfair and unjust rank, because it just won't be fun, there's no way I'm going to get out of bronze, and even if I do it would probably take the whole year, then I would just get placed there again.