Free useable pool space not releasing

[Krextyl]

Prior to my system migration not long ago I deleted a bunch of incremental client image backups I had stored on my pool. Fast-forward to now and I noticed my TrueNas is reporting a sizeable amount of space still being noted as used despite that prior deletion. Most of which - some 10Tb, coming from one particular path. Below shows the WebUI reporting in contrast to the windows folder (using root/full access) to report the folder content size. What am I missing here? Please forgive my inexperience - how do I update the dataset to correctly reflect the true size? Does it have to do with prior snapshot backups or something similar that I need to purge/update somehow? I would not be surprised that the old content isn't still there until some other copy-on-write, overwrites it but I would have thought that the UI would reflect the size as now free for other use....

Note the I5-2500K size of ~10.6Tb vs it actually containing less than 1Tb of information just as windows is reporting in the lower image.

 

[sretalla]

zfs list -t snap Storage

Do you find any snapshots?

 

[Krextyl]

From UI I have 756 snapshots (counting recursives) going back from the middle of this year, if I do the command above from shell I see 21 snapshots having the same bookend dates