Lava doesn't decrease BlockSkyLight Level, while water does that. Because of the fact that you cannot see through Lava this is very weird. In the screenshot you can see the player fully submerged in lava, but apparently still in clear view of the sky (full skylight)
Water does decrease skylight 3 per block.
Environment
MacOSX 10.11.6 Java 8 Update 101 NVIDIA GeForce GTX 680MX 2048 MB
Affects 16w33a