Arena tournaments match you up randomly because if two people are ready (but not if one of those players is someone you just played against in your previous game) they are immediately paired up. If someone was rated 2000, they could still be paired up with you.
They try to pair you up with someone your rating, but if there isnt any, then you will be matched up with the person to whom you have the closest rating but havent played against yet.
I'm 600 elo and I was matched up against a 1100 elo player in my 4th game in chess tournament, why?
I was playing in a 10+0 arena tournament in chess.com.