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, and Drivepool will kick off a manual re-balance. When it's done balancing, toggle that plugin off again. If you have any doubts about whether or not the pool display is correct, just force a re-measure before starting the manual re-balance.