Ranking Is Not Valuation: Selection-Aware Carbon Claims under Supplier Shift

Cement production emitted about 2.4 Gt CO$_2$ in 2024, and specifying lower-carbon concrete is a near-term procurement lever. Yet material-search tools often use predicted embodied carbon twice: first to rank candidates, then to quantify the selected product's saving. We evaluate that reuse on 108,326 ready-mix concrete Environmental Product Declarations (EPDs) from 413 mostly North American manufacturers, holding out whole manufacturers. Ranking retains useful signal under supplier shift while valuation degrades: mean within-class pairwise accuracy is $0.591$, but mean log global warming potential (GWP) $R^2$ is $0.082$. Selected mixes average 10.4% lower declared GWP than their class median, yet their declared GWP averages 9.1% (about $31\text{ kgCO}_2\text{e/m}^3$) above the value reported for them. We derive a conditional identity linking this claim gap to search depth; pools of 2-50 products recover its linear prediction ($R^2 \ge 0.97$). Changing the selection rule buys little, at a cost in realized advantage; replacing the point claim with a selection-conditional conformal floor keeps the chosen product and cuts overstatement above 0.05 log units from 51% to 11%, though that floor overshoots by about a third and still misses its 10% target on 15% of unseen-supplier cases, so it too needs validating. The result motivates a rank-then-report architecture for carbon recommenders. Code: https://anonymous.4open.science/r/ranking-is-not-valuation.