Placements are broken?

I just played my placement matches with my friend, we both started together and played together each game, we lost 4 and won 6 in a streak. Anyways, as we finished, we both found out something strange(?). As we played every game, we assumed we would get placed in the same division, but NOPE(!?!). I got placed on Silver 5 while he got Silver 2. So I am asking, why did we get such a huge(if you ask me) difference? It is illogical, isn't it, same games, duo, different ranks? I'd appreciate if a rioter could reply on this, rather some random dude who thinks he knows stuff. Thanks. Chris
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