Clock appears darker when upside-down in item frame

I had placed a Clock in an item frame some time ago, and recently found out it got darker if I flipped it upside down. It will also get darker on its side but to a lesser extent.

What I expected to happen was...:
I expected the clock to remain as bright as it was.

What actually happened was...:
It got darker. Not the whole item frame, just the clock itself.

Steps to Reproduce:
1. Put an item frame down, then put a clock in it. (Note that I had it on a Wood block)
2. Rotate clock to either side or upside-down.