I gotta say even two of the same class are not always a good comparison. My two warlcocks which are VERY similar in gear vary wildely in dps from recount. I gave up worrying about it. It just seems with classes that are crit dependent damage meters are not a good rule of thumb due to the luck of who is critting.
Also something I did notice last night was that lag can also affect things badly. I had a lot of trouble killing stuff last night and finally just went to bed, everything seemed to take so long to kill. First thing this morning I went back and ran a fast dungeon before work and nuked everything in seconds, my only conclusion could be network lag.