Why are the Enhancement Metals so similar?

[Nicrosil]

Aluminum is internal pulling, and removes the user's metal reserves. Duralumin is the internal pushing, and burns the user's metal reserves in a single, super powerful flare. Chromium is the external pushing, and removes the touched target's metal reserves, and Nicrosil is the external pulling and burn's the touched target's metal reserves in a single, super powerful flare.

Why are the internal/external pairs so similar? None of the other quadrants are that similar. The internal physical metals enhance strength and senses, but the external physical metals don't enhance other's strength/senses. Copper and bronze are wildly different from zinc and brass. Gold shows you your past or alternate self (personally I think shows a change in Identity, but that's a theory for later); it's not like cadmium shows you another person's past... that's what malatium does... huh.

Is there a reason for this? I know Harmony and Preservation changed around Allomancy, is this an artifact for that, or just a weird coincidence?

[Weltall]

Given that Brandon has said that he set up allomancy based on powers that would be useful in a caper story and then built the system from there, the Doylist explanation is probably 'because that spread of powers works best for storytelling purposes'. And remember that the classification system is an in-universe attempt to categorize the magic to the best of their ability and it works pretty well there and for our purposes but that doesn't necessarily mean that it's a perfect description of how it works. So not all external metals might be opposites of the internal metals, not all pushing metals opposite of their counterpart pulling one etc.

[Oversleep]

Just weird coincidence.

Out-universe: IIRC Brandon just decided they'd fit like that:

Quote

the others don't have 1/1 correlations. But I liked this concept far too much not to use it.

https://wob.coppermind.net/events/243-hero-of-ages-qa-time-wasters-guide/#e5265