These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth, always in the same order: nitrogen-phosphorus-potassium (N-P-K)

Shouldn't that be "Nitrogen, Phosphate and Potash?"

Nitrogen = N
Phosphorus = Phosphate = P (Technically, phosphorus is an element (P), and phosphate is a molecular anion (PO4_3-), part of phosphoric acid (H3PO4). In (medical) diagnostic testing, they're often used interchangeably, sometimes called "inorganic phosphorus." It's also an essential plant nutrient, so on fertilizers you'll see the phosphorus content expressed as a percentage (by weight) measured as P2O5, the anhydride of phosphoric acid.
Potassium = Potash = K ( from latin kalium)

In order to determine the actual weight of P & K in fertilizers, an extra step is required. A fertilizer formula reports phosphorus as P2O5, not actual phosphorus (P), and potassium is reported as K2O, not actual potassium (K). To convert P2O5 to P, multiply the listed P2O5 % by .43, and to convert K2O to actual K, multiply the K2O value by .83.

3:1:2 ratio fertilizers actually supply nutrients in a 3:.43:1.7 ratio, which squares rather nicely with the average actual NPK usage.

Your fertilizer's NPK RATIO is a much more important consideration than its NPK %s.

Al