Not really my area of expertise, but that value is the calculated lighting value of your block.
Your crucible has noOcclusion which allows light to pass through it:
https://github.com/NovaMachina-Mods/ExNihiloSequentia/blob/cb9490413242e4480551433299127e9066243a7a/src/main/java/novamachina/exnihilosequentia/common/block/crucibles/WoodCrucibleBlock.java#L24
Your barrel does not:
https://github.com/NovaMachina-Mods/ExNihiloSequentia/blob/cb9490413242e4480551433299127e9066243a7a/src/main/java/novamachina/exnihilosequentia/common/block/barrels/WoodBarrelBlock.java#L23
so it is probably not letting light into your block according to the shape?
Look at how ModelBlockRenderer.tesselateWithoutAO uses LevelRenderer.getLightColor() of the neighbouring block/side as the light level when light cannot enter the block.