Your understanding is correct: volumeBindingMode belongs to StorageClass. The Helm chart's templating is likely using this variable to configure an associated StorageClass, and it's not being directly applied to the PersistentVolumeClaim in the final manifest.

If you find volumeBindingMode actually appearing in the rendered PVC YAML, then that would indicate an incorrect Helm chart definition, which should be reported to the chart maintainers.