It is different, but it is logical, because in the movie you see many of the Witch'servants among narnians and they would not have fond memories of Peter.
It's isn't logical that Peter would have forgotten what it means to be High King in the space of a year.
One of my favorite books in Frances Hodgon Burnett's
A Little Princes about heiress Sarah Crewe, a little girl attending a fancy boarding school in London while her father, the rich and dashing Capt Crewe, is in India. Sarah is so splendid, her friends call her Princess Sarah, and she tries to behave as if she reay were a princess.
When her father died penniless, and word gets to the boarding school that Sarah is now poor as dirt, the evil headmistress takes al Sarah's things and clothes, makes her into a slave, starves and mistreats her. Yet little Sarah, for the first time in her life hungry, cold and lonely, determines she will behave as if she were a princess, anyway.
When she gets her hands on some fresh bread, although she is starving, she gives most of it to a beggar. When her former classmates who are so insensitive they don't even realize she is hungry and exhausted, come to her for comfort and help with their studies, she puts her own grief aside and helps them.
She recognizes that being a princess isn't just wearing gowns and having servants. It is an attitude of fidelity, perseverance, of royalty.
Peter in the book knew this attitude very well. It would have made his character in the movie very noble and a true hero. Instead, we were needlessly given a Peter whose character immediately fell apart at the first signs of adversity. Little Sarah Crewe puts him to shame.
Between Peter and Caspian, in the film, Caspian is much the more royal and stronger character.