Scales. Whats the resolution of your scale?
I use one that has only 1gram increments. This lead me to believe I can be as much as .49 grams off when making a recipe. For example, 2% salt for a 400gram recipe might be 7.51 to 8.49 grams. Worse, if I want .75% instant yeast, that's 2.51g to 3.49g.