Spending on a material can decrease because it changes the price, even if the amount used grows. Understanding the use of resources requires a distinction between economic values and physical quantities. The two measures are both useful, but cannot be automatically interpreted as equivalent.
I propose to register an understandable unit and a stable perimeter. Packs, pieces and weight can represent different aspects. The comparison must take into account the amount of activity carried out, avoiding reading as an improvement a reduction that depends only on a lower volume of work.
In a hypothetical example, a firm buys a material at a lower price and bears a lower expense. The quantity for each product may have remained the same. Separate reading clarifies the economic benefit without ascribing to it an unobserved physical reduction.
Verification of measures requires consistency in records. Further elements and appropriate methods are needed to estimate environmental impacts. Distinguishing spending and quantity makes the discussion more precise, allowing for recognition of what results have been achieved and what conclusions remain to be verified.