I'm with Giacomo in that specific NPK ratios for certain plants seems to be mostly marketing but there may be other micronutrients and elements included that benefit certain types of plants. Iron, magnesium, sulfur, calcium, pH adjusting additives, etc.
The plant doesn't care what the NPK ratio is on the bag. It only cares about whether there are enough accessible nutrients in the soil.
One popular approach is the "sufficiency level" approach. If your soil has enough potassium for example, you don't need to add more potassium. Nitrogen gets used up pretty quickly and leaches out of the soil with water so the addition of that is usually required.
Soil testing by a good lab will tell you exactly what's in the soil and provide recommendations on how much N, P and K to add based on what you tell them you're growing. You're better off having your soil tested than trying to guess which bag's NPK formula is best for you.
Check your local university's cooperative extension office if you're in the US. You can use out-of-state labs as long as your state is not on their exclusion list. I found the prices at UConn soil testing lab reasonable.
How much nitrogen, phosphorous and potassium the plant actually winds up using, and how much washes away is going to vary based on different growing conditions. I believe fertilizer manufacturers formulate their NPK ratios based on generalizations they think will work best.
If you really want to provide optimum nutrition to your plants it's worth doing a soil test every 1-3 years, add the recommended lbs of NPK based on the results and keep track of what you add and how the soil changes over time to see if the plant is getting everything it needs or if you're adding too much of something and it's building up in the soil. You may notice that phosphorous keeps increasing so you can cut down on the amount of phosphorous you add.
If you don't want to do soil testing you can just pick a bag of a fertilizer you think is good and see how the plant responds. Various nutrient deficiencies (or sometimes excesses) will cause physical changes in the plant or different results that you can see. Make adjustments based on that.