I'll speak about what I know. I play a Vanguard and I am usually somewhere near the top of the damage charts.
It comes down to 2 simple things really. I am partially ranged, one of which is a ranged low damage long duration DoT. There is never a GCD when I am not hitting someone.
Second, I have a passive DoT that is applied to my attacks. I also have talents that refresh them. I have DoTs ticking like crazy all WZ. That is where the numbers come from.
There are times I will be at the top of the WZ in damage with 400k, the next person on my team has 300k. The difference is I had 10kb's and he had 17. That is a better indicator.
It is mostly fluff and smoke and mirrors.