Illusory superiority is a psychological phenomenon that causes people to overestimate their ability in a particular task or life skill. Everyone does it.
Two scientists, Dunning and Kruger, conducted a study where they had groups of people perform a variety of tasks and then rate themselves on their competency. Every group declared that they were better than average - even the lowest performing groups. In fact, it was the lowest performing groups that rated themselves the highest. They found that the worse you are at a certain task, the better you probably think you are.
This principle has been proven to be true in education, work, athletics and more. 87% of Stanford MBA students rated themselves as better than average. Obviously, this is impossible.
The long and short: everyone thinks they are better at things than they actually are.
Clearly, MMO's don't escape this phenomenon. In fact, they probably exacerbate it - especially PVP environments where a person is put into direct competition with another "worse" player.
The thinking goes like this:
1. I just lost to this operative/BH/Mara/Sent in a WZ
2. I am a better than average player so it must not be me
3. NERF!
The thinking should go:
1. I just lost to this operative/BH/Mara/Sent
2. I am most likely an average player
3. How can I better counter that class next time
Basically...
L2P