Labels on platforms such as GitHub support triage and coordination, yet little is known about how well they align with code modifications or how such alignment affects collaboration across contributor experience levels. We present a case study of the Kubernetes project, introducing label–diff congruence—the alignment between pull request labels and modified files—and examining its prevalence, stability, behavioral validation, and relationship to collaboration outcomes across contributor tiers. We analyse 18,020 pull requests (2014–2025) with area labels and complete file diffs, validate alignment through analysis of over one million review comments and label corrections, and test associations with time-to-merge and discussion characteristics using quantile regression and negative binomial models stratified by contributor experience. Congruence is prevalent (46.6% perfect alignment), stable over years, and routinely maintained (9.2% of PRs corrected during review). It does not predict merge speed but shapes discussion: among core developers (81% of the sample), higher congruence predicts quieter reviews (18% fewer participants), whereas among one-time contributors it predicts more engagement (28% more participants). Label–diff congruence influences how collaboration unfolds during review, supporting efficiency for experienced developers and visibility for newcomers. For projects with similar labeling conventions, monitoring alignment can help detect coordination friction and provide guidance when labels and code diverge.