Guild Wars 2

Well, to be precise if it has a drop rate of 0.005%, then the chance to get at least one in 20000 runs is still only about 63.21%.

Also 10 people running the event daily for 7 months is 2100 runs.
That means about 10% chance that at least one will be dropped.

If the drop rate is 0.05% those numbers become about 99.995% for the 20000 runs and 65% for the 7 months example.

According to the “unique chest” theory and taking into account the frequency most events with a named chest are completed, I believe the steam ogre in the Thaumanova reactor seems very propable to be the source.
I believe most people have done it once to get the achievement if at all, since the way to fight it is not so apparent and many may not bother to charge the key repeatedly to farm it.

As far as the staff not making sense to drop in such a place, I guess because of the nature of the reactor anything can end up there.