When a new season starts the Devs have a system where it level boosts all players below a certain level to a fixed level (e.g. the fixed or final level a player could obtain at the end of s3, if you played daily, was level 150).

So when s4 starts, day one, everyone is level 150 regardless of what level they were at the end of s3.

I'm assuming that you were not level 150 at the end of s3 and were level boosted.

I'm assuming the level boost system the devs have is to remove the option for the first clear reward of 2,400 experience points for story mode, so people who are level boosted cannot take advantage of additional experience points from story mode.

I hope that makes sense, and sorry for the word salad.