Perception is reality, and everyone's perceptions are different. Sometimes sort of.
Characters build their own personal mythologies, both about themselves and each other.
And when the harsh light of day shines through, it don't expect to see a pretty picture. If this book were a movie, the audience would be shouting advice to the characters on screen. Pullman enjoys examining the nature of reality in a completely different way than the most literature geared to younger readers. We don't have to go to another planet, time, or dimension to see a different world we just go to another character's point of view. The theme of betrayal, especially unintentional betrayal, appears in a lot of Pullman's work, but in The White Mercedes , it is the story's mainstay.