Exact same placement matches give different ranks

Recently my friend and I decided to play our placements together without being ranked in previous seasons. However, after going 9-1 in our placements, he was put into gold 1 while I was placed into silver 1. Naturally, this led to some confusion. My friend believed the reason was that whereas he played a variety of junglers, I mainly played Zed in the mid lane. Does this actually have an effect on my MMR or final rank? If not, then what caused him to be placed an entire division above me?
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