Ah yeah I forgot you have a pally - thats a huge difference... think of it this way (and this is really dumbed down but you get the idea)

If you're doing 4000 dps with a paladin tank, having a DK tank would make your shamans do 4000 * 1.13 = 4520 DPS - thats an increase of 500 just for having a dk tank The rest could be attributed to gear, technique, lag, etc.