The GUI is showing the raw space available to ScaleIO as in "how much disk space you have given it", in your case 6*900 = ~5.4TB. As your data is essentially duplicated, if you create and use 1.6tb of a thin-provisioned 3TB volume, then the raw space that will take from your pool is approx twice that, so 2x1.6tb = roughly 3.4tb.  Look into the scli --query_volume command for more details on how much space is used, but if you're wondering why it's not just using 1.6TB then see above and know that you don't get a single-instance copy of your data. That never happens in ScaleIO, it's always duplicated. Mirrored, whether you like it or not
I have a ScaleIO 2.0 deployment in my lab.  There are 6 SDS servers with 900GB each.    I created a single 3TB volume, and presented it to a single linux SDC.  Nothing else is currently using ScaleIO in my environment.  The volume is part of a volume group.  This particular logical volume uses XFS.  It is the only logical volume in the volume group.  According to the linux server, I am using 1.6TB of the 3TB volume.  In the ScaleIO gui, It shows the following  5.3TB Total 1.8TB Spare 3.4TB Used?????  Protection level and other settings are default from the install.  Can anyone help explain where the additional space has gone?  Thank you.
ScaleIO using twice as much space as expected