In Walk Two Moons, why does Phoebe keep having to make up new lies daily?

Sharon Creech's novel Walk Two Moons is a story about a young girl named Sal. Sal, or Salamanca, is a girl who goes on a road trip with her grandparents to find her "missing" mother. Throughout the trip, Sal shares stories with her grandparents about her friends and her life.
One of Sal's best friends is a girl named Phoebe Winterbottom. Phoebe is an interesting girl who often lies and exaggerates. Phoebe also has a wild imagination and tends to judge other people frequently. One of Phoebe's lies is that Sal's father's new girlfriend is actually an axe-murderer.
While Phoebe is an extremely loyal friend, she does have personal issues that are the underlying reason that she lies. Phoebe's mother is missing. Phoebe convinces herself that her mother has been abducted so that she does not have to deal with the idea that her mother could have willingly abandoned her. Rather than coping with this, Phoebe begins lying to cover up things about her personal life.
Phoebe and Sal help each other come to terms with many of the hardships in their lives.