I have a Dell PE R520 with 18.1 TB drive (built on RAID 6, disk internal to server).
I setup Microsoft's Deduplication functionality on this drive and began moving large amounts of material onto it.
Now it shows that it has .75 TB free out of the 18.1 TB; but if I pull up the properties of each of the top-level folders on this drive they show 24.47 TB as the uncompressed file size and 4.66 as the actual (deduped) size on disk.
The 24.47 TB makes sense - since it is deduping it is able to squeeze more into less storage.
What doesn't make sense is why there is only .75 TB free if the deduped files are only consuming 4.66 TB.
What am I missing?
You are most likely missing the dedup folder itself as it is hidden and the permissions make it impossible to get into by default.
These chunks are stored under System Volume Information\Dedup\ChunkStore on the drive in question.
The sizes you see outside this location are the non duplicated data.
Edit - link describing the chunk store and mismatching folder sizes.
https://blog.workinghardinit.work/2015/06/08/windows-deduplication-and-mysterious-folder-sizes/
Just as a suggestion you might also want to stick with using the 'Volumes' view under the File and Storage Services tab in Server Manager. As you've found the UIs don't match each other.