The trench shot in question was not simply two takes that were fused. The “first half”, before the laser flash, was a still image, and that was transitioned to a model shot with the flash to cover up the change. This is why the black levels are different, and the color in the SSE just does a better job of covering it.

EDIT: For the record, this is my second favorite shot in the scene, right behind the first person shot of Wedge’s X-Wing flying toward the TIE firing. Oh, and the still image is easy to see if you look at the warping at the edges of the frame that occurs.