Why isn't it? Is that drop integral to your game experience? If you hadn't Googled the boss beforehand, would you even know that that drop was in the boss' loot pool? You've gone out of your way to expose game mechanics that the game doesn't explain, and now you're complaining that you don't get the specific weapon that you had to Google to know was in the game. I have no inside information beyond what the developers have said on the forums. See my answer above - you're not meant to know what gear the boss can drop. The game isn't designed for you to be able to predict the drops enemies give you.