Playing good -> winning -> winrate rises
Farming damage -> PR and XP rises -> more often in the top 3 players
But the important part:
Playing good ≠ Farming damage
To win, you have to play for the objectives. Get caps, focus important targets. Important targets are mostly those with less HP, like DDs. Which means less damage for you.
So basically, you will do less damage if you play good, but get a higher winrate. And you are doing the opposite. Farm more damage while ignoring the real objectives of the game.
This is the reason for both, the winrate drop and PR raise.
But that's exactly what "random" means. There will always be fair matches with equal teams, matches where you can do what you want and will still lose and matches where you can basically be afk and still win.
And of course their will be random streaks of any of these three.
If you really want to examine the fairness of the matchmaker, you have to look at a very large number of matches. That's how statistic works.