Short answer: It is a feature.
This is the effect of either Intel's "Display Power Saving Technology" or AMD's "Vari-Bright" feature, the intent is to reduce power usage by reducing brightness when you look at dark images (which ostensibly would not need to be back lit so much). This technology detects whether the screen is displaying a mostly dark or mostly light image, and adjusts the back light accordingly. Unfortunately this can be annoying if important parts of the UI are light colors (like Chrome), or when dealing with highly visual applications (Photoshop, Media, etc) where changes to brightness have a negative effect on visual fidelity.
So yes, this is a feature, unfortunately this means either dealing with the effect, or forfeiting the power savings it produces.
If you dislike this feature, it can be disabled (or adjusted) it in the settings for your graphics card.
The exact layout of Intel's options may vary from system to system, but on my machine:
Intel system tray icon > Graphics Properties > Power > On Battery > Display Power Saving Technology
It shouldn't be too hard to find, even if it isn't in the same place on your machine. You may need to look for the properties in control panel.
Open the Catalyst Control Center (search for "CCC" in start, choose CCC - Advanced) > Graphics > Power Play > Enable Vari-Bright [Check-box] [Source]