Why am I matched with a 1400?

I've been getting into chess and have been playing quite a bit lately. My rating is about 580; it bounces back and forth. When I start a new 20-minute game, sometimes I'm matched with 600s or 700s (who I can barely beat), and sometimes with 400s. However today I'm starting to be matched with 13 and 1400s. I don't see how it's fair to match a person with 500 elo with another with 1400. That's a difference of 900 elo.

Your rating range for seeks is probably set to any below and any above your current rating. 

You can modify that:

https://support.chess.com/article/1962-how-do-i-choose-what-rating-my-opponents-are

This happened to me when i played 20 minutes aswell. Because it's not a common time format, the pool is smaller and it's harder to match players. Try going in a bit more popular time format

Thanks, I've changed the setting. Hopefully it works. Are you sure Chess.com won't match me with anyone with a higher rating than the setting specifies if it takes too long?

No, they will leave you sitting there unmatched for as long as you can stand it. 

That's why some people who like to play the less popular time controls broaden the range of ratings they will accept.

Yeah I eventually switched to playing 10 minutes mainly and started getting better matches. I also began to play even shorter time controls.