Help - 888gb storage being used by system

Hi

I have a 2011 27in Imac running Big Sierra 1-.13.4 with a 2tb SATA drive and a 3tb external drive for time machine back ups.

If you see the image you will under more info about system the size looks strange ( Size; 9,174,916,499bytes 6.29gb on disk) and in the details for the drive 888gb it shows being used by the system folder and the disk inventory anaylsis shows the system file as only 8.5gb.

Has anyone any ideas what is happening.

Many Thanks

Nick