Here's why:

* The pH scale is logarithmic: This means that each whole number change in pH represents a tenfold change in acidity or alkalinity.

* Lower pH values indicate higher acidity: A solution with a pH of 3 is ten times more acidic than a solution with a pH of 4.

Think of it this way: the lower the pH, the more hydrogen ions (H+) are present in the solution, and the stronger the acid.