Mark, thats what drop rates are. There is a percentage that an item will drop when you defeat a boss. And so all the percentages for a drop are added together and depending if probability favors you or not you will or wont get your desired gear.
I have some things to reply that might help a little. 1.Did you bring 2 to 3 players along.If so then you might have a more chance of getting that gear and get more loot.(More loot)=more chance. 2.Its not just that you get gear you don't want.Its supposed to drop different class clothes with maybe the same drop rate.So if you were farming for it then you could have a unlikely chance to get it. 3.Did you stay around for quite a time to wait for Zeus to reappear?If you didn't do that then that might be a problem.This could work on several or some bosses.But the spawn is only 1 time so bring friends if your teammates left. Logan Darkwraithlvl51.