See if I'm looking at this right:
I'm trying to collect all the WvW siege decorations for my guild. They are only available via RNG from the WvW Siege Decoration Case skirmish chest reward. There are 21 possible drops from this chest. Obviously we can't say for sure what the drop rates are, but it seems reasonable to assume they are equal, therefore it's roughly a 4.8% chance to get any one particular piece each time you open a chest (trial).
I'm currently missing only one piece - the Guild Ballista. I have opened 177 chests. If my calculation is correct, the probability of getting at least one success (the Guild Ballista) over 177 trials is 99.983452215%, and the inverse, the probability of NOT getting one over 177 trials, is 0.016548%. Makes my situation seems ridiculously unlucky. Is my math right here? Is it just bad RNG or is this item inexplicably rarer than the others? Also why is this box RNG at all?