Because, like most pills, there is a significant amount of filler material that isn't the medication in order to make the pills larger, bind together into a pill form, etc.
When making these sorts of things if you don't have a consistent homogenization of ingredients you will have some pills with too much of the medication and some with too little. The margin of error in fentanyl is very small and the people making the pills don't have the same quality control as say an actual oxycontin factory would. When they have hot pills come out people die even if they are taking their usual dose.