Overwatch [official site] devs have patched the game to stop that situation in competitive play where opponents could leave mid-match if they thought they were going to lose and reduce your Skill Rating gains at the end of a match to a sliver of what they would have been. The only thing is, the community is still reporting that the issue exists.
The idea that led to the problems was that when you're playing competitive mode the game calculates the probability of each team winning the match - it's part of working out how fair or even the match is. That then gets used after the match to adjust the measurement called your Skill Rating. If the game was a tough match-up and your team wasn't expected to win then winning will net you more in the way of Skill Ranking gain and you'll lose less if it goes badly. The reverse is true for the favoured team in the matchup.
"Previously, if a player from either team left mid-match, the game would automatically recalculate each team’s probability of winning, taking a look at not only who left the match, but also when they left. The assumption was that if a team was a person down, the odds of them winning would go down, too, and that the most accurate thing we could do was account for this when adjusting each player’s Skill Rating at the end of the match."
Thing is, that works when people are using the system in the way it was intended, but knowing how the systems of a game work impacts how people tend to use them. Thus you could reduce the winnings of another team by leaving a match partway through forcing a recalculation of the odds of winning. Heck, even if it was unintentional and you just have a dodgy connection it could still feel a bit rubbish for the winners, knowing that their win was worth less in the way of Skill Ranking just because of someone leaving.
Blizzard ended up deciding it was more important to combat the incentivised negative behaviour than enforce the accuracy of that particular system so Skill Rating gains or losses will be set according to the probabilities set at the start of the game.
The post continues:
"From a technical perspective, this will make Skill Rating adjustments a little less accurate for that specific match, but it should ultimately feel better for the players who stay and finish it out (plus, your Skill Rating will correct itself the next time you complete a match with no leavers). We also hope this will remove the incentive to “troll” a winning team by leaving, since it will no longer have an impact on anyone’s Skill Rating."
That post was actually from last week but I'm writing about it now because Blizzard have now confirmed that there was a bug so the change didn't actually come into effect until a day later, hence a whole bunch of confusion about what was going on and whether the change had happened or not.
Players are still reporting that leavers are having an effect on their Skill Rating gains though. Reading through the posts it's an interesting situation where either bugs still exist/the change isn't working as intended or there are other factors at work that are contributing to that perception of unfairness.
I remember interviewing Jeffrey Lin from Riot and he told me: "there was a time when we nerfed a particular Champion and saw the win rate of the Champion plummet a few percentage points. However, we found out after that the nerf didn’t get implemented into the patch. The simple fact that the patch notes described a nerf to the champion had a weird psychological effect on the play patterns of players and resulted in a drop in win rates."
The Overwatch update post notes that other information is factored into those competitive match calculations :
"There are other factors that still have an impact on Skill Rating, though. Things like your team’s probability of success, your individual performance, and win/loss streaks can all affect how your Skill Rating is adjusted at the end of a match. These factors are there for a reason (to help make Skill Rating as accurate as possible), but we know that it’s not always clear right now why your Skill Rating is adjusted the way it is.
"That's a concern for us, because we not only want Skill Rating to be accurate, but also for it to make sense. The fact that it doesn't is good feedback for us, and seeing how we can make Skill Rating feel better/be clearer is definitely something we’ll be keeping in mind as we look at ways to improve Competitive Play for future seasons."
Without having access to the Overwatch data it's impossible to know which situation is actually occurring, but given it's leading to frustration and negative behaviour regardless of the cause (i.e. if people are leaving games because they think they can reduce their enemy's winnings REGARDLESS of whether that's actually true) I'd say Blizzard need to find a way to offer more transparency about the way at least some aspects of competitive play are communicated.