It's all about math really, so I'm not sure if you would want me to explain it.

The reason is that, two cold gems do not multiply each other in the damage formula, they add up with the native scaling. Whereas percentage gems do multiply with each and then strengthen both on the base damage and scaling part.

If the native scaling is too high, then adding up two more times on the scaling may not gain more than multiply twice on the whole base+scaling bonus.

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And, Ludwig's Holy Blade has the highest native arcane scaling among all pure physical weapon. After some calculation, we found that it is the only case that requires only one cold.

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Check https://www.reddit.com/r/bloodborne/comments/7as4zh/how_to_calculate_ar_w_blood_gem_effects/

for more info.

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Or the short answer: It does not need two cold because after trying all combinations of gems in the game, we can actually see it has the best AR with one cold and two elemental.