Varying LP gain, how does it work?

I recently started playing ranked games with a friend I introduced to league a couple months ago. We duo bot, myself adc and him support. After going through his placement games where he was placed in bronze 1 while I remain stuck in bronze 4, we discovered that he gains more LP and loses less LP through ranked games. Can someone explain what is causing this? Is it his higher status, the fact he is support, because he just came out of placements, is he likely getting better letter grades, or something else?