It depends on last season ELO and WHICH games u won.
For example u get a higher elo if u win the first 9 and lose the 10th instead of losing the first and winning the last 9.
And also if u had high silver last season e.g. silver II, I u may also get platin V.
A friend of mine had 9/1 win/loss and got gold I. (he had high silver or low gold in last season)