Neil,
this can occur is fabricpool aggregates with external tier.
run the below command for answers. look at Footprint in Performance Tier & Footprint in external_tier
cluster01::*> volume show-footprint volume_01
Vserver : Vserver_01
Volume : volume_01
Feature Used Used%
-------------------------------- ---------- -----
Volume Data Footprint 18.48TB 14%
Footprint in Performance Tier 6.34TB 34%
Footprint in external_tier 12.41TB 66%
Volume Guarantee 0B 0%
Flexible Volume Metadata 142.5GB 0%
Deduplication 32.63GB 0%
Temporary Deduplication 9.46GB 0%
Delayed Frees 281.6GB 0%
Total Footprint 18.93TB 14%
Regards,
Mani