Not only does this result make sense, but I could also see it as something baked into the S5 itself. Considering screen brightness on devices (phones, laptops, watches etc) is supposed to go to the absolute lowest in a black environment, it could just be coded to provide a lower brightness level when the ambient light sensor detects certain darkness to save battery over time.
So, personally, for me, this would be welcomed and I wouldn't actually ever even notice it... Unless...
A photo in blazing sunlight would be a better indicator of this... Of course, then a lower brightness would be an issue outside during the day. I'll have my S5 in a week and will be comparing this too just for fun.