Reduction on its own is not a result: returning nothing reduces the shopping space by 100%. These reductions are reported only alongside the recall and ranking quality measured on the same runs, which shows the reduction preserves substantial reference relevance rather than simply returning fewer options.
The reference ordering is price-led, while the production objective is multi-factor: seasonal suitability, reachability from the origin, stated preferences and affordability together. Recall and NDCG therefore measure agreement with that reference rather than traveller-fit quality directly, and 100% agreement is neither expected nor necessarily desirable. The gap contains both cases where Discovery is right to prefer fit over cost and genuine ranking errors, and these metrics do not separate the two. Read them as evidence that relevant options survive pruning and rank highly, not as a score to drive toward 100%.