Perhaps Terajuma Island is supposed to simulate the Alolan climate? In the case of Sun/Moon, Charjabug evolves in Vast Poni Canyon due to its unique Magnetic Wave Field. Also, Pikachu can only evolve into A-Raichu on Terajuma island which seems to further corroborate the idea. Since we happen to have a Power Plant in West Gearen, perhaps it was intentional to split Charjabug and Magneton's evolution spots? After all, it makes sense that Charjabug would evolve from a natural phenomenon since it's a bug type, whereas Magneton being a steel type would require an artificial phenomenon.