Match making

SO everyone says match making is based on mmr and skill lvl. How come i have so many games were my teams average death is 8+ and my average is 3? My average cs at 10 is 60-70 while my teams lately have been 40-50? How are all the players with such bad KDAs and low low low CS on my skill lvl? All my games are completely 1 sided. Either 30-5 or 5-30. RIOT please fix you lower tier match making its garbage!