4 hours of 2x drop at Cold Eyes

So, everytime you kill a WS you have the 1/X chance of getting the ilbi. Nothing says you have to get that ilbi after you have amassed the X amount of kills, therefore you will never really get a good aproximation of the drop rate.

Really, when talking about droprate, we mean the average. Everything has a finite average. That includes ilbis. It's just whether or not someone wants to spend the time there and get data for over a x amount of ilbis (because as the amount that you find increases, so does the accuracy of the "droprate")