I had to justify this design choice in a paper I wrote about osmolarity, here is the answer: "The skin should be removed from the potato because it is the main barrier to water loss. When the potato is in storage the skin’s chemical composition, structure and/or thickness can change. This is notable because it will affect the water loss/gain during osmosis, (Hol). Since we do not know the storage conditions of the potatoes (these conditions can often affect the skins ‘quality’) they might all have different skin compositions etc. To eliminate this unknown variable and ensure fairness all skin must be removed from the potatoes used."
Study refranced (Hol): Hol, Stefan. “Modelling the Physiological Changes of the Potato Skin Regarding Water Loss during Long Term Storage.” Feb. 2018.