Skylight does not behave as expected

What I expected to happen was...:
I build a large square of 50 by 50 blocks at y-level 319. I expected it to do either one of the following:

• Nothing because it is too high and the skylight will spread from other blocks.
• To have the light level consistently drop while going closer to the centre of the square.
• To have no skylight below all of the blocks under the square.

The last case is the one that I hoped would happen, but I expected either one of these three to happen.

What actually happened was...:
Inconsistent patches of light below the square. Which are not in compliance with either one of the above cases (see screenshots).

Steps to Reproduce:
1. Build a large square at a high y-level
2. Look down and see weird patches of light.