Light is everytime on 15

I was playing Skyblock and was wondering why mobs aren´t spawned.
Then I was going into the debug mode and saw that the light is always on 15, but it was night.
In the nether should everything normal. And when i was in the netherportal the light switched to 14.
The screenshot is not at night, because I missed to do it at night, but it is raining and then it isn´t 15 as well.

Hopefully it is understandable! German language skills