Lemon is my go to example for explaining this. 
The acidity of lemons varies wildly, so any amount given in a recipe needs to be adjusted after tasting the lemon, or, if possible, the dish/mix/sauce for which you are using it. For coctails, this is especially critical. 
With, let's say, 20% of the drink being lemon, the result can be disagreable sour, or bland, and it is all your fault. Not the lemon's. 
Taste, taste, taste.