League Promos

Alright. So today I finally made it through all of my qualifications for a league ranked status. In my overall opinion (and the opinion of people I've played with) I have been doing very well in these ranked placements. I did fairly well in all games I played (excluding losses) and i managed to win 7 of my 10 placement games, that's 70%. However to my surprise when my last game ended I was placed in the lowest of the low ranks, Bronze 5. I don't even begin to understand why I've been placed this low seeing as (from what I've heard) the highest you can be placed from your promos is Platinum 1. Now you'd only expect to get that high if you won all 10 of your games and were a flawless league player, and No I'm not asking to be put in that high of a tier. But you would think that if people get placed that high for winning all 10 games, then why do I get placed as low as you can go for winning 7 of my games? This event is very discouraging to me as the player because I no longer see the point in trying so hard in my placements if you can reap no reward from them for winning as high as 70% of them. In all honesty I now see why people don't take ranked as seriously as I do, because they've probably been suckered by this system too and been placed so low for no reason what so ever. I would really appreciate a reply to this thread about why I was placed so low or what benefits you can actually take from winning your promos seeing as I was obviously given nothing for my efforts.