Value is less than the recommended default of 6813825536 (Reserved space in bytes per volume. Always leave this much space free for non dfs use) . Why was the setting not set to the default on install.
This may be a case of configuration defaults not being in sync with recommendations for configuration defaults.
During the installation process your cluster will be assigned certain defaults to start. As you move through cluster setup, you'll have the opportunity to change those defaults to the recommended configuration defaults through dialogs that throw warnings and errors onto your screen - it sounds like your administrator may have moved past this step without editing default values to match recommendations.
Another possibility is that your administrator has changed configuration settings after cluster setup, but did not change dependencies of those settings based on recommendations.
In either case, you should be able to change these fields to their recommended values if they make sense in your environment and pass basic sanity checks.