Buttons create a very tiny amount of light when placed

Placing a button in a very dark area appears to make the block it was placed on slightly lighter. The difference is very minute, but it's a difference.

What should have happened was:

The light shouldn't have changed.

What did happen was:

It did change.

STEPS TO REPRODUCE:

Find a very dark area, preferably somewhere with a light level of 2 or less.
Place a button, and watch the block become slightly lighter.