So how does someone in platinum able to play in silver tier

It might be a bug thats been around since ranked S2.. If you finished in Platinum and got reset to Silver, your first match will seed you in a Platinum match, even though the game recognizes you as a Silver. I was Diamond with my 2nd account ranked season 1 and that account had been reset to Bronze4 now but when I played my first game with it, I was displayed as a Bronze4 together with my Master and Diamond 2 randoms =) They were pretty upset about it haha.

Anyhow, ranked is total mess and probably the worst design Ive ever experienced in a serious "competitive" pvp game. I am last split D3 and just started to play ranked this split in Plat4 and at least 50% of the times I get killed, its a current Diamond4 or higher, with last season Predator or Master badge etc. But whats really funny is how I NEVER see any Gold4 players in my games. So it seems like low Platinum players are likely to get matched against everything above, but very rarely against players below....

And dont get me started about the hordes of Smurfs that exists in EVERY RANK from Bronze to Platinum....This game is designed to make people smurf but thats another story.