Unable to account for 23.6 GB of disk space
Both About this Mac and df seem to agree that you have 91GiB used, 21GiB free on /, which adds up to the volume of your 112GiB (120GB) hard drive.  

The problem must be with the list of files Disk Inventory X scans. 

Run du -s as a superuser starting from / and then going deeper into the filesystem to get a more accurate picture of your disk space usage.  



Keep in mind:
About this Mac and Finder display space in GB (base 10, 1GB = 1000MB)
df -h and du -h show results in GiB (base 2, 1GiB = 1024MiB)
df -H and du -H show results in GB
Many GUI tools display file size in GiB while labeling it as GB