Skipping Ranked Divisions

Okay, So I was Silver 5 when I got promoted to S4, and had MMR Equivalent to S2. But I kinda dropped to S5 and had MMR of S3 then. I played my prmos and won it and yet I was promoted to S4. Why didn't I skip S4 as I had MMR of S3 according to op.gg

P.S. I am from SG servers, id is xoBloodMercyox