I lost the amount of LP I gain from two won games, for leaving a game that I won for 10 minutes.

This morning I logged on to league to play some ranked and I had 41 LP. I then went on to win two games but I left the last game for about 10 minutes because my team won. Because of my leaving I got a champion score of D- and I lost 21 League Points. I understand leaving should be punishable but what I don't understand its why I'm getting a D- in champion score and losing the amount of LP I would lose in two lost games in a game I obviously had an effect on. I played Shen top with teleport and went 4/3/10. Because of some unexpected events I had to leave the game for the last 10 minutes of it and i come back to find this. I think that if you leave, no matter what you should still get punished, but losing so much progress in ranked and getting a stupid bad champion score for leaving a game for 10 minutes, a game that you had an impact on and that your team won anyways, should not come with this punishment.

Match history link: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1926355275/214381387?tab=overview