[milesvp]

Blizzard blogged about this some years ago with regards to their SC2 matchmaking.

They found that their matchmaking was too good, and would cause a lot pf stress as people were constantly feeling pressure to play their best since the matches were so even. This led to very short play sessions as players would feel exhausted from one or two matches. Their solution was to increase the standard deviation of the algorithm, so that you had a better chance of roflstomping your opponent and a similar chance of being stomped. They claimed it led to much longer play times and less player stress.