So being able to one-shot somebody once every 3 minutes does not mean you are able to one-shot. What's this logic? I don't even....whut?!
The logic that Arena rating follows is "the more wins you get => the better skilled you are" so it's normal to face better players when you put some wins in, especially if you haven't played for a while. But after you get stomped, it reduces the rating requirement of your opponents, in the attempt to face you against players of equal skill. If you play a few dozen games, you will realize that you have reached a point where your win ratio is close to 50%, which would indicate that the rating system has put you in the rating pool of your capabilities.