Zoology 10th Edition by Stephen Miller,John Harley

Exercise 18

The subphylum Myriapoda is divided into four classes. The millipedes (class Diplopoda) possess two pairs of appendages per apparent segment and are round in cross section. They inhabit leaf litter and feed on decaying plant matter. The centipedes (class Chilopoda) possess one pair of appendages per segment and are oval in cross section. They are nocturnal predators. Lesser-known myriapods include members of the classes Pauropoda and Symphyla.
Why is it important to use the phrase "legs per apparent segment" when describing the millipedes?