Brandon Sanderson (paraphrased)
As it turns out, there is an error in the Feruchemical table when Brandon put it in Mistborn 2. If you look closely, Determination (electrum) doesn't belong in its group. The group that it is in is obviously more physical powers. Determination was supposed to be a mental metal, and Warmth was supposed to be in that Physical group. He just made a mistake originally. But it turns out that Feruchemy obeys different rules than Allomancy, so Brandon isn't retconning it, but saying that Feruchemy works differently now. Apparently there was going to be a table of Feruchemy at the end of Alloy of Law, but it wasn't ready because Isaac kept thinking like an Allomancer. Feruchemy has its own rules (for example, Brandon confirmed that pewter does steal Feruchemical health, probably because that second group of physical Feruchemical powers are also "physical", so pewter can steal them.) Hemalurgy also obeys different rules.