Getting different amount of lp for a win in flex queue as premades

Hello, I was playing flex queue with a friend and noticed that after a win he would always get a few more lp than me (e.g. I get 23, he gets 26). So I was wondering why is that? What does the amount of lp you get depend on? Some important information: this is the first time we played flex, after the placement matches we naturally got the same rank as well. Also, he has a rank in solo/duo queue, but I don't. Does your solo/duo MMR play a roll in this? Anyway, because of the different amount of lp we get he was granted a free win in his promos after our last game and I'm stuck at 97 lp [sg-janna]