[Nick Cohen]

Hi there, this post is a bit old but I was wondering the same thing and found a reply elsewhere. My understanding is that, while a cross-dissolve will equally fade one clip out and the other in, an additive dissolve will combine the luminance values creating a spike in luminance at the mid point. Comparing the two transitions while looking at the video scopes seems to confirm this. I guess it makes for a punchier transition in some circumstances.