Some players despite them having prior clears are weakest links, but usually it doesn't make a big deal in a group that is able to make up for that weakness (especially with overgearing) whether it's pug or static. When such players then go to join random parties and end up getting matched up with possible other weak links does it then be more apparent that the group may not get very far unless everyone actually knows what they are doing with very few mistakes. If a group wipes early several times, the experienced players often leave because they know if it's a lost cause whilst the weaker ones bail knowing they weren't going to get a simple carry. Well carry probably isn't the correct word as everyone needs to play a part, but you get the idea of what is meant by it.