"df -h" is showing about 1/2 "Available" disk that the "Disk Utility" shows for same internal SSD. Which is correct?
They’re both correct in that they are measuring two completely different things.

Disk Utility is measuring the volumes on the disk itself whereas df is measuring the file system.  To draw an analogy, one is calculating the surface area of the dining table and the other is measuring the surface area of the plates.

So, the important bits here are the available space.  Disk Utility is telling you that the disk itself has 120GB of available space.  df is telling you that on this particular partition/volume you only gave 53GB available.  

So, the space isn’t missing, its just not allocated by the filesystem.  How do you claim it?  Create a new partition, resize an existing partition, etc.