When viewing the Online Storage Capacity Detail report it can be seen that not all free capacity is included. There are some aggregates that are not included in the calculation.
This is happening because underlying view is expecting flex volumes to be always created from aggregates. The missing capacity will come from aggregates with no flex volumes creates on them. If there are aggregates with flex volumes created then the free space (if any) from that aggregate will be included.
This issue cause originates in CommandCentral Storage and CCER reports on that information.
There is a hotfix available for this issue.
CCER 5.2 RU1