I'm actually starting to wonder about this myself. One of my FC mates ran Aery around 40 times yesterday and the only tank armor piece that dropped the entire time was the chest. I initially wrote it off as just really bad luck, but then today I ran Sohm Al somewhere between 20-30 times (I've lost track at this point) to try and get the tank set from there. Still haven't seen the tank chest or hands, only one legs and boots, and two helmets. And on the other end of the scale there've been 10 AST weapons, 2 sets of DRG gear, 2 sets of BRD/MCH gear, and 1 set of BLM/SMN gear.

I dunno, maybe it is just bad luck but I think it's possible the RNG might be weighing some sets more heavily than others right now.