I feel like my play has no outcome on the game...

First off, 4 man premades in both of my games today. How fun for me (sarcasm :D). Anywho, 1st game all lanes losing I'm getting 3 and 4 man dove mid, 14 to 6 with all of our turrets and 2 inhibs down and we suddenly just win. Out of nowhere, I guess we killed 2 of them then proceeded to sweep but it all happened so fast...Anyway it felt stupid to win when I did nothing to contribute and the other team admittedly played us like a fiddle for 20 minutes.

Next game, we're crushing! I camp top and start off our lee sin top 3/0, rampage around to bot and before 10 minutes were up 7 - 1, two more 4 man sejuani ults put us 2 teamfights ahead and we secure baron. We start sieging mid when their maokai gets caught and a fight ensues, we get swept and lose the game. Right there. Again, I'm not sure what happened, but it felt like their team just hit a point when their comp won the game, and the previous game vice versa.

TL:DR I think games of rather equal skill leveled players are either 20 minute ff stomps, or lost in champ select, and I don't have a say in them either way.