Look at Vader's Belt Buckle - Like an elongated "D" shape... Throughout the film it is a "reversed" "D" shape except in a few shots during this fight scene... To prove they have mirrored the image, watch which hand is dominant on the lightsabers for both Obi-Wan and Vader... They start the fight with their right hands on the upper - dominant position. "Right-over-left" if you will...
In the scene there are several spots where suddenly they are "left-over-right" and it is obvious (by examining the features on their clothing) that the entire clip was flipped over. I think this has more to do with the staging and dynamics of the dominant position of the fight than anything else...
--Xo