Additional from
http://oz.wikia.com/wiki/Margaret_Hamilton
You may recall that Margaret Hamilton was burned during the exit scene because the flames started before she was completely below the floor. This happened on December 23 1938. She was not able to return to work until February 10.
It's quite possible that the close-up was filmed after February 10, and either the set had been struck by then, or her distance from the house was different, putting it out of focus.