Here's why:

* The magnitude scale is logarithmic, not linear. This means that a difference of one magnitude corresponds to a consistent brightness ratio, not a simple arithmetic difference.

* The brightness ratio for a one-magnitude difference is approximately 2.512.

Therefore, a magnitude 1 star is 2.512 times brighter than a magnitude 2 star, which is 2.512 times brighter than a magnitude 3 star, and so on.