I remember a few days ago I was playing competitive overwatch when one of the people on my team randomly DCed after we completed the first half (we were on Kings Row and we got the payload to the end). He comes back a minute later and we hold the enemy team to the second point. Everything was normal until I got to the screen which shows your rank move up and down after a match. It only moved an inch even though we won fair and square (playing 5v6 for like a full minute too).
Overwatch's ranking system is bad. It focuses too much on team performance and too little on individual performance. The only way to really go up in rankings is either to be just THAT good or only play with a full team of people you know. Also, the coin flip shit is fucking terrible.