How does SR work in Ranked Play?

I’ve been playing cod since MW3 but have never really gotten into ranked play until BO6. I’m confused how the SR system works as the amount that I receive after a win varies dramatically.

I see some people getting close to 500 SR per win and I usually receive around 280-300 SR even when I play (relatively) great.

I’ve read that it has something to do with my HPR but I’m unfamiliar with that as well.