You need to use the Pre/Post multiply checkbox on the CC node.
When you adjust things like brightness, this can introduce negative values for example. Or push values up, even if the alpha is 0.
Gamma will keep lower and upper values untouched which is why you don't see that reflected in the merged image.
It's generally a good idea to use the Post multiply checkbox to avoid edges with possibly problematic luminance.
You could also divide the alpha prior to the CC, then multiply it again after the CC and have the same effect.
This post by Bryan touches this topic as well:
http://www.bryanray.name/wordpress/blac ... he-basics/If you're a beginner you should definitely check out his posts.
While I'm not a Fusion beginner anymore, not a day passes in which I don't mourn the fact that he isn't writing on his Fusion book anymore. I can't imagine how much valuable information he still has to offer.
(He still offers incredible amounts of information here and on WSL, but a complete composting handbook by him would still be incredible)