Dan Radcliffe isn't just Harry Potter - he's an actor in other things too. In 2007 he worked on Order of the Phoenix, My boy Jack and the December boys. The last two wee period pieces where he was required to have short hair.

Sadly you cannot grow hair to order - hence Harry's long hair from G.O.F. had gone!

As he was very young when the first film was made - surely if you have qualms about his acting style you should blame the director? Dan was probably only doing as he was told!