Enterprise Vault only monitors specific file types under the cache location. One of the file types not monitored is temporary files which are used as data is being written and constructed, which can result in the cache limit being exceeded.
Enterprise Vault is working as designed.
Allow for additional disk space use, above the Cache Size Limit set. How much extra would be unique for each environment and entirely depend on the utilization of the cache area, however as a rule an additional 50% of the Cache Size limit would be sufficient for most environments.