Glicko-2 is the MMR used by Arenanet in pvp. It is most commonly used in chess. The problem in GW2 : it does not include the number of games played.
Thus, if you are a good player with a volatile RD, or a medium player, well you can always have a fresh player into your game.
It literrally happened to me two games ago, my MMR is maybe medium, but I don’t really wanna have players saying “what happens if we kill lord” in my team.