Looks like it might be the angle of the light hitting the 'micro-texture' of the slats. Even in the second photo, you can kind of see the differences. I think it is byproduct of a manufacturing texture left behind in the same way a sawmill leaves behind small kerf marks during milling. I think...
Just plug in a portable HVAC in the outlet in the vault. That way you're not spending energy heating or cooling the cabin when no one is in there.
You can use one of these. I have the "EnjoyCool" model...