The pH scale is not a normal straight-line scale. Each step of 1 on the pH scale is a change of a factor of 10 in hydrogen ion concentration.
When the hydrogen ion concentration of a solution increases by a factor of 10, the pH decreases by 1. When the concentration falls by a factor of 10, the pH increases by 1.
This means a solution of pH 3 has 10 times the hydrogen ion concentration of a solution of pH 4. A solution of pH 2 has 100 times the concentration of a solution of pH 4, because the difference of 2 pH units is 10 × 10. Each extra unit of pH difference multiplies the change by another 10.