They work as intended. Base impurity is always 1 and impurity from each crafting slot gets multiplied. Example, you have 3 slots:
No premium slots: 2 imp, 3 imp, 3, imp, result: 18 imp
1 premium slot: 1.5 imp, 3 imp, 3 imp, result: 13.5 imp
2 premium slots: 1.5 imp, 2 imp, 3 imp, result: 9 imp