Greedy Goblin has a good post today titled 'The fact of the bad group'.

Quoting: "With 2 hard mode kills I'm a fairly good WoW player, up in the top 5%. Your chance to bump into me at a random Zandalari heroic is exactly 0 as I never go with a non full guild team (actually I don't go at all). On the other hand the worst players are rejected from every teams, so they are overrepresented in random groups where they cannot be booted instantly after their first stunt."

There is a lot of truth in that statement, many good players avoid using the lfd tool and run with premade groups.