System taking huge amount of storage in my SSD - macOS High Sierra

Try running Omni DiskSweeper:

https://www.omnigroup.com/more

DiskSweeper can give you a more accurate read of disk space than Finder or Storage Tab of About this Mac. Itcan also show you the precise size and location of all your files. It can also give you a more accurate read of disk space than Finder or Storage Tab of About this Mac. Itcan show you the precise size and location of all your files. It will inventory your disk starting from the files that take up the most space. It can also give you a more accurate read of disk space than Finder or Storage Tab of About this Mac. If you want you can even delete files from OmniDisk Sweeper. But be careful some of the items may be important to your system.

That's because your Mac is reporting your storage space incorrectly. If you've already tried re-indexing Spotlight and the other methods, the only other possible solution I can think of would be to reinstall Mac OS. But if it isn't causing any issues, I'd tend to leave it alone.