Unequal drop rate between Zombie and Spirit event.

My experience mirrors a lot of others; way more spirit than zombie.

Without a large enough sample size though its difficult to know for sure if it's bad luck and there is an equal chance for either to drop.

There is no reason that it has to be equal though. And it really wouldnt suprise me if it started equal, but then after the game population had done the "beta testing" for them, they tweaked it to favour spirit given that zombie turned out to actually be useful.