why did David Vincents hair change on mirror earth?

His hair should've been parted the same way when he got to mirror earth (the way Thinnes usually parted his hair in the first half of the movie and in The Invaders etc )

Obviously what happened is for the second half of the movie they inversed the footage but they should've taken this into account when shooting and specifically styled Thinnes' hair the opposite way so when the footage was inversed his hair would appear in the usual way