What happened to storage space after removal of an APFS volume on High Sierra?

Running 10.13.3 on late 2016 MBPr w/ 2TB SSD. I wanted to create a separate bootable space to run Sierra so started to create a partition through Disk Utility but it suggested a "volume" instead so I made a 30GB APFS "volume". Tried to install Sierra and it did not show the volume as an option when booting from Sierra installer USB drive. Back to Disk Utility and removed the volume but my main "Macintosh HD" volume that previously said I had 2TB of space total now says 1.97TB of space, which is exactly 30GB less than before. Volume is gone and SSD has all 2TB but the space associated with the volume, previously part of my Macintosh HD, is no longer accessible to that volume. What happened to it and how do I get it back?