Why does the NPK values in fertilizer differ from one fertilizer company to another for the same type of plants? For example, one company has a fertilizer for tomatoes and the NPK numbers are different from another company that also has fertilizer for tomatoes.
Let's assume that fertilizer companies compete with each other by trying to offer a product that meets their customers' needs, like any business. There is a range of needs for different plants and soils. Where I live, there is a lot of phosphorus naturally; we look for fertilizers with low P values to avoid the detrimental effects of excessive phosphorus. Vegetables need much more nitrogen than most other plants. Container gardeners have to meet all the nutrient needs with fertilizer because they are not growing in soil. I'm sure the manufacturing process plays a part. Each of the major nutrients is a chemical compound, not pure N, P or K. Each compound they consider using will have different costs to produce, result in different strengths which affects packaging size, shipping cost, and on and on. How it is intended to be used is another manufacturing consideration; will it be hand broadcasted, worked into the soil or carefully placed around the base of a plant? Formulation decisions consider ease of use for each method; this is why the majority of ingredients are "inert". Pure nitrogen can be purchased but one has to be very careful using it. It is easy to give too much and plants then suffer nitrogen root burn. And if it worked in past years, you bet gardeners are looking for something similar this year. On the other hand, if a formulation wasn't commercially successful, the company will change it. Organic sources are much weaker than chemical formulations which explains the relatively low NPK values to organic fertilizers. I hope I provided some insight into the manufacture and marketing of fertilizers.