Availability not match with actual used in Extreme SSD

I am using the SanDisk Extreme Portable SSD 1TB, it shows my available storage is 52GB.

However, I only used 246 GB, here is the breakdown
246.1 GiB [##################] /Extreme SSD

Where is the rest storage gone? It is an 1TB hard disk but I only used 246 GB and 52GB left, may I ask why?