It was just the way the special effect was done at the time. Because the scene takes place against a white background, the matte lines (the black lines around objects in old effects shots) would have been more visible. They got around it by making the layers more opaque.

The entire scene was digitally recomposited in 1997 for the special edition, so no black lines, no opacity. This was used for Harmy's 1.0.

But because Harmy is trying to make the despecialization as original as possible, he added the opacity back in with 2.0.

Hope that makes sense!