The value 66 TB displays the total size of the data in the backup job. We get that value from the CommSever database.
When you click on the group name and see the charts that display 14TB, that size is coming from Analytics Engine, which is showing the total size of the data that we have analyzed.
This means that some jobs have failed to process and hence the mismatch. Please let us know if this makes it clear, why you are seeing this huge difference.