duc indexes multiples of the size the indexed disk is large

I run duc with

sudo duc index -xp  /

I chose these option to ensure no file system boundary is crossed - however with a root disk / of 500GB duc prints out the following progress messages:

.
.
.
skipping /System/Volumes/Data/home: Not crossing file system boundaries
Error statting .Spotlight-V100: No such file or directory
Error statting .installer-compatibility: No such file or directory
Error statting .PKInstallSandboxManager-SystemSoftware: No such file or directory
Error statting .TempReceipt.bom: No such file or directory
skipping /System/Volumes/Data/home: Not crossing file system boundaries
^C---#---] Indexed 2.7Tb in 60.1M files and 11.5M directories

How is that possible? Indexing 2.7TB on a 500GB disk? Should I have let the indexer finish?