It's most likely due to the way the visor was installed. The real helmets are asymmetrical and just very nasty all around.
The Jedi helmet was painted at the same time as the Prepro 2 helmet and another Prepro 2 style helmet prior to the filming of ESB. So, time was not a factor. I think they just grabbed a helmet and went with it.
Besides, it's not until the excellent tour photos that we've even been able to tell that the mandible is crooked. As JFJ said, film hides TONS of imperfections!