understanding fertilizer a little better

[1999frontier]

I've got a question, when they say you want 1lb of N per 1,000 sq ft, you take 100/(first number on bag of fert.) and you get how many pounds of product you want for each 1,000 sq. ft. So why does it matter if the N number is 18 or 30. You will need less product if you use a 30 nitrogen bag.

18N you need 5.55 lbs of fert per 1,000

30N you need 3.33 lbs of fert per 1,000.

Same goes with P. So I guess my question is, is there a difference if you use a 18-24-6 vs a 30-24-4 and use the calculations?