Inconsistent Ranked Reset Placement Across Seasons

For the past 3 seasons or more, I've been reset from Platinum all the way to Bronze, while my friends start in Silver or even Gold. 

We always play ranked together, and I consistently get more damage (1,500–2,000), kills (3+), assists, and support as Lifeline. My teammates usually get under 800 damage and 0–1 kills. Despite this, we earn almost the same RP every match, and I have to spend most of the season catching up to them.

Why am I consistently being reset much lower than my teammates when we play the same matches and I perform better?

Is this a bug, or is there something wrong with my account's ranked placement?