Some seem to forget the boss or monster having limited lifepoints!
If one member doing awesome DPS then one other can NOT do high DPS because the monster is already dead or skipping phase or whatever...
If all doing 1k in savage its the "norm" but when you have one doing 1500 then there must be an other just doing 500... you get the idea?
There can be a lot reasons for "one" to do more damage than others (AST buff as example) but i recently seeing that happen a lot, special if ilvl 240 players are pairing in dungeons with ilvl 220 players...

I just like to say: the so called "weak" or (as you like to call them: afk) player, may do all right and still cant compare because the other one have taken some of his damage...
One Example from experience: BLM casting while MNK is killing the target before cast finished -> MNK getting all the honour for DPS and BLM is kicked for being lazy...
(Sidenote: I know some who said they do it on purpose and have fun when BLM could not cast, gives them an edge of thrill)