Light Levels in debug screen perpetually equal 15

I noticed this many versions back when I was trying to figure out lighting for a large room. Even if it is night time, if you open the debug screen with F3 and look at the sl and rl values, they always say "15".

Edit: Although this has not been updated since 13w06a, it has existed in all intermediate versions through this update (14w04a). Another screenshot is also available in MC-45473, although the bug there is different.