“I understand that one is judged on one own bechira, but why does that mean he gets the same sechar?”
Because the entire purpose is that in the olam haba community no one will be able to point to “his” or “her” fellow and say this neshama gets more reward- it’s not just since they had it easier in life or they had more opportunity. Thats why the reward is purely given out based on ones bechira and no more.