It's because we're still using oldschool Elo, just with some weird decay addition. In short, if you don't play for some time you lose elo, but it is quickly restored when you start playing again. This is so you come back and are forced to play the game regularly if you want to remain high on ladders. (Not up to me whether this is morally acceptable..)
But there is a savior on the horizon,
WHR has been implemented on the test server and is awaiting deployment (eventually). Due to requirement to behave like Elo, it will also lower when you are away, but once you come back it'll try to reevaluate your skills instead of simply returning your old Elo.
Come and join my religion.
Also go throw some 1v1s, it makes team games playable
