Someone who was Silver 4 last season finishes placements with Plat 4?

So I was playing Ranked Solo/Duo yesterday and I came across a Silver 4 player on my team. I'm already puzzled on how a Silver player even popped up in a Plat game. It's one thing if the guy is a smurf and is really good but, this guy has been Silver for 4 seasons in a row and from I witnessed that game, his skills reflects his rank perfectly. I took a break after that game as I was pretty let down. Once I came back, I decided to search him up and see how he fared on his placements (he was still unranked when he was in my game) and I found out that he just finished it and got placed in Plat 4......wtf? So how does someone who was placed in Silver 4 last season get placed in Plat 4 after going 6 wins 4 losses in placements?