Matchmaking in this game is beyond shit, in fact it's possibly the worst experience I've had in an online environment, totally discouraging for new players. A friend and I started playing 2 weeks ago and are now finding ourselves matchmaking v's people with 600-700+hrs playtime or more, whilst everyone on our team is seemingly sub 80 hrs.
That's a massive experience disparity and we aren't learning anything other than this a waste of our time.
There are lots of factors to take into account for the matchmaking. If you're grouping with friends then you're not in the solo queue. One major factor is you're probably being queued with and against friends who have wildly different MM ratings. If a High tier player plays with a beginner, you end up with really weird matched games.
There's really nothing that can be done in that instance without doing something drastic like preventing people from playing with friends. I suspect that in the games with the 600+ wins there are probably people with sub 10 wins or something silly.
And as others have mentioned, if you're win rate is still 50% against these people then maybe they are on your level?