The actual reason Dorothy's hair is different lengths throughout the film is not because of different wigs but because it took over a year to shoot the film. Her hair was at different lengths throughout different scenes because movies are not filmed in sequence.
Stop pondering this slip up because I have the answer to this question. Movies take months, sometimes years, to complete. During this time, the actors' appearance changes, therefore pictures are taken at the start of taping. Every time they tape the picture is taken out and used as a reference to make the character look exactly the same. However, during the Wizard of Oz they did not take the picture. For this reason, whenever they taped, Dorothy's hair grew out and became longer. Perhaps they cut it sometime so it looks shorter.
This film was made on a very low budget, in a very limited period of time and it was plagued with problems from the start. Characters were cut and the film went through a whole series of directors each adding their own input to the way Dororty should look. This is why there may be a few slip ups when it comes to Dorothy's appearance and several directors shot footage of her different looks. Anyway it's a great film made ahead of it's time all slip ups aside!
It's because her hair was only shoulder length, and they used all different wigs. They weren't sure what length of hair to use. They also mixed some of the scenes together so that's why. In the first two weeks of taping her hair was to be blonde and curly, so they didn't really make all their minds up at one time.