Grow big is fox farms single grow formula and meant to go with their tiger bloom for a cycle grow. The 3 part formula is meant to be mixed at different levels to achieve total control over your grow. You can mix different amounts and get different levels of NPK. NPK is just the base nutrient you add boosters and other nutrient enhancers after you have the NPK set. (Other wise there is no point in calling them enhancers if they are not extra) There are calculators that can tell you what the NPK should be by mixing what amount of each part of the 3-part formula.

Each time you mix your 3-part formula you are not always getting the number on the label.
What mixing chart are you using?