For some drugs, the government only has to prove the weight of the substance no matter how much actual “drug” it includes. In other words, if “crack” is cocaine, baking soda, and water, its irrelevant how much of each is included – whatever the “crack” weighs, that is the weight you will be charged with.
On the other hand, some crimes look to the pure drug weight itself. For mushrooms, the government must extract the psilocybin and determine its amount, rather than just weighing the entire mushroom itself.