From what I understand, the 1C scaling comes from the Mythological Mystic Code Servants (Nero, Tamamo, Nameless, Gilgamesh) scaling to BB, and the rest of the servants scaling to them. Since the more powerful servants are usually nerfed by their servant containers, a servant's true power is being scaled to their Mythological Mystic Code counterpart. However, the Mythological Mystic Code is exclusive to the Moon Cell for one, and for two, it doesn't awaken the power that the servants had in life, but the latent potential of their origin concept. Their origin concept is vastly superior to their living self. Even then, the Mythological Mystic Code servants barely stood a chance against BB. These same MMC servants are vastly stronger than the other Moon Cell servants.
IE: Moon Cell Servants only scale to 1C because Mythological Mystic Code servants scale to BB and the rest of the Moon Cell Servants scale to them. Regular Servants scale to Moon Cell Servants because Regular Servants are nerfed by their Servant container and Mythological Mystic Code Servants are awakened to their true power. However, the Mythological Mystic Code is exclusive to the Moon Cell and it awakens the power of a Servant's origin concept, which is vastly superior to their living self. Said MMC Servants barely had a chance against BB, and regular Moon Cell Servants don't stand a chance against them. On top of that, with the Moon Cell being a simulation of reality, loopholes and bugs exist. Furthermore, BB and Kiara don't have 1C durability, they simply possess 1C hax.