Items in cache reported on Vault Cache Properties page may show less than expected when compared with the number of items shown in the vault store usage report for the archvive. This may occur even when there is plenty of available cache size and the Current status shows Complete.
This can occur when there is a transient BITS error and multiple content cache databases (.db files) for the same quarter (3 month period).
This issue has been addressed as part of the following release:
Enterprise Vault 9.0.4 Cumulative Hotfix 5 Release
Enterprise Vault 9.0.5 - Release Details
Enterprise Vault 10.0.3 - Release Details