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...
I'm planning on putting mine up for $300 - $350 a day, but with restrictions. On-road only. Local to my county. High deposit. Full insurance coverage only, Cleaning deposit to cover atypical soiling (puke and stuff). I'll use a tracker.