size discrepancy between replication source and destination

[guermantes]

So I replicated my snapshots of /mnt/TANK/dev to /mnt/OFFSITE-GREEN/backup/dev. The number of files and directories in source and destination are the same. The number of "dev snapshots" are the same. The compression and record sizes are the same for both datasets. But there is a large discrepancy in the disk usage sizes. What might that be due to?

Briefly during the Replication task process, in which the task was actually replicating two datasets dev and sysadmin, I saw the orange Running button flicker into a red Error button and then becoming orange again.

Code:

root@freenas:~  # ls -R /mnt/TANK/dev | wc -l
  309640
root@freenas:~  # ls -R /mnt/OFFSITE-GREEN/backup/dev | wc -l
  309640
root@freenas:~  # du -s /mnt/TANK/dev
11684743    /mnt/TANK/dev
root@freenas:~  # du -s /mnt/OFFSITE-GREEN/backup/dev
10660803    /mnt/OFFSITE-GREEN/backup/dev