The novella the movie is based on (Rita Hayworth and the Shawshank Redemption by Stephen King) is different, and while a lot of the adaption was done with a broader movie-going audience in mind, many of the choices they made were to give closure and complete character arcs.Really creative.
There's alot that bugs me about that movie, tho. Like two or three rock smashes opens that big of a hole in the pipe?
Also, theres usually not pressure like that is a drain pipe to cause a geyser when Andy breaks it.
Maybe I've just seen the movie too many times.
There is a great breakdown on Mashable about these differences. Worth reading the novella if you haven’t already.