Not the same frames sent to the same machines... problems with a few frames during rendering so that they had to be recomputed at the end... PC doing other things on the network during rendering (updating antivirus, getting big mail, defragmentation). As the rendercows are low priority, computing time can easily be eaten by other apps. So a lot of possible reasons, and as it is a practical experiment done in real conditions, I did not want to disable all background programs running.