They tend to polish up characters for the movies, any movies. Remember how Hermione had hugh front teeth (like a beaver) and wild bushy hair? and they gave her perfect teeth and smooth even waves (i don't think JKR actually mentioned any curls, just hte bushyness).
Ron was suppose to be skinny and tall with a large nose and nobody would call Rupert Grint's button nose anywhere near large and he isn't ridiculously lanky.
Also, harry's hair was supose to be sticking up in all directions, but they just gave him messy bangs.
I just chalk it up to plain Hollywoodification