I was making a mob grinder completely out of cobble stone. There are no openings for light to enter and no torches left behind and F3 still shows Sunlight level of 15. The whole inside is visibly brighter than what it should look like and isn't just an F3 display error. I've tried restarting Minecraft and trying to update the light with torches but still no change. Mobs will not spawn inside the grinder, so the light is actually there.