[JLV]

It seems to me, based on the reading I've been doing, that perhaps the answer lies more in the nature of the iron itself.

From what I've been able to gather, it seems that Steel lacks the attributes of "cold iron" for whatever reason. It occurs to me, based on that, that smelting the ore in order to make steel (including coking and the much higher temperatures required for the entire steel making process), somehow renders the iron no longer suitable for the purpose of resisting magic. Whether that is because of the "impurities" added in the steel-making process, or because of the excessively high temperatures required in that process, I don't know, but in either case, the iron is no longer suitable.

Similarly speaking, Bronze is not Copper -- it is a smelted metal requiring a mix of impurities and different temperatures to allow for its creation by a metalsmith. As a result, bronze is not used for ceremonial purposes, whereas copper is frequently mentioned as a ceremonial metal.

Based on these admittedly speculative thoughts, it seems to me that PURITY might be more important than magnetism or anything else. "Cold iron" might therefore simply refer to pure iron, smithed to make a weapon or tool (such as a ceremonial dagger, for instance), but not smelted to create a stronger, more durable material. Similarly, Copper could be used for some functions (though being a less dense material, it might lack certain properties of iron), simply because it was smithed into a useful shape, but not smelted into bronze of some other variant of the metal.

Meteoric iron might therefore be more effective simply because it is a "purer" form of iron (in game terms, not necessarily "scientifically" speaking) already purified by the nature of space itself -- whatever that might be in your fantasy universe -- and therefore better for anti-magical purposes.