If you reboot a node, the Health Processor (HP) cycle stops in the middle of its iteration. When the node boots back up, it will continue where it left off. This means that reporting of Used space (and trapped space, thus free space) requires you to have completed at least 2 HP cycles after the reboot in order to present the correct usage.
You can check the HP cycle via SNMP or by drilling down into a node under Node Operations.
There is a script in the Tech Support bundle (detailed here: https://support.cloud.caringo.com/tools/Tech-Support-Scripts-Bundle.pdf) called snmp-castor-tool.sh which may be helpful.