How can all players in the Starcraft 2 Grandmaster league win more than they lose? Starcraft 2 is a competitive online strategy game where players compete in leagues with other players of similar skill. The most difficult and highest league is the Grandmaster (GM) league, which contains the top ~$200$ players in a region.
The matchmaking system will, most of the time, match players from the GM league with other players from the GM league. There are exceptions however, like if no one else from GM is playing, or if someone from a lower league has a very high MMR (Match Making Rating) but is not in GM for whatever reason. The algorithm is quite complex, and as far as I know not all details are even public.
For the purpose of this question, let's say that whenever a GM player is matched with someone from a lower league, the GM player wins that match.
These are the current standings in the GM league for the American region: http://www.rankedftw.com/ladder/lotv/1v1/win-rate/?f=am,grandmaster
You can see that everyone's win/loss ratio is higher than 1 (more than $50\%$ won), so everyone wins more than they lose in GM. The standings change often, but it's rare to see anyone with more losses than wins. Wins and losses are counted from when you start playing, not only from when you entered GM. However, stats are reset a few times per year, at the start of each season. So I would expect this not to influence things too much.
This is rather weird for me to see: I would expect the worse GM players to be, in general, easy pickings for the better ones, and their win/loss to be below 1.
One explanation that I can think of is what I call low transitivity (if there's a proper term for it let me know): if $A$ consistently defeats $B$ and $B$ consistently defeats $C$, then it rarely holds that $A$ also consistently defeats $C$. In such a case, all 3 players $A$, $B$, $C$ can hold similar win/loss ratios, but I still don't see how all 3 can hold them above 1.
Under the assumption I mentioned above, that a GM player will always defeat a lower league player, it's possible that they are all above 1, but it still seems highly unlikely, since inter-league matchups are quite rare.
What is a possible explanation for this phenomenon? Given those win/loss ratios, what is an approximation of the number of games a GM member will play with lower league players?
Without the assumption that a GM player will always defeat a non-GM player, can we say something about the probability of that happening?
 A: This is an older question (from 2015) but your point about "if GMs are matched with non-GMs, let's assume the GM wins" itself explains the phenomenon.
There's only 200 players in GM league, so it seems incredible if all games were GM vs GM (as if you have 200 players at any time, it's rather difficult to find a match on demand).
Even if only a portion of the games were against lesser-skilled opponents, that would be sufficient to have a >50% win rate.  
So the GM league in general has a >50% win rate, and the aggregate pool of non-GM players in general have a slightly <50% win rate (although the non-GM pool is so large that the win rate can still be only a tiny bit under 50%).
In general, b.net's match-making cannot guarantee perfect matches at the top or bottom of the ladder.  Top players (e.g. GMs) generally win a lot of games due to being matched against lesser-skilled players (due to no one else being available), and the worst players (e.g. bottom of bronze league) generally lose a lot of games due to being matched against stronger players.
