This should explain (source):
Maruti is a highly popular name used to address Hanuman. The literal
meaning of the word 'Maruti' is the one who is born from 'Marut',
meaning air. The other names that suggest Hanuman is the son of Air
god include Maruti Nandan (Air god's son), Marutadmaja (the one born
from Air), Vayu Putra (Air god's son), and others. There is an
interesting story how Vayu or the Air god became Hanuman's father.
Essentially, Marut refers to air, and Maruti can refer to Vayu (god of air) or Hanuman (born from/of air or Vayu). In the name of Maruti-Nandan, Maruti refers to Vayu.