Were all of the drives empty and added to the Pool before files were placed on it?  Perhaps some of the files were manually copied into the hidden Poolpart-xxxxx folders in the root of each?  Hard to tell without a screenshot and knowing how you populated the pool.    What you can do to even things out, is install the Disk Space Equalizer plugin for Drivepool and force a manual re-balance.  You go into "Manage Pool", then Balancing, then enable it in the Balancers tab.  Hit Save after that,