It's done like that on purpose 🙂
Swiping the cc up darkens the rest of the screen, while the cc remains the same. When you adjust the brightness, it adjusts the entire screen (including the top part which is darker), so you can get a good idea of what your brightness level will look like.
In iOS 7 it didn't do that, so whatever brightness level you picked, ended up being a bit darker (when you closed the cc), which annoyed lots of people😛