I don't necessarily think it's her skill increasing as time goes on, I think she's adjusting the level of her writing to coincide with her characters becoming older.
JMHO.
JMHO.
I had to read all four books TWICE to my kid! It was pretty enjoyable. My poor wife is lost when me and my son discuss the characters, and subplots of the movies, since we know the stories frontwards and backwards. The movies are as true to the book as they can be cramming so much into a few hours.