There are a few different things in play here, but unfortunately @CGreenTX is right. This is pretty much the way of the world now in modern macOS. Below are some of the key things that contribute to this oddity.
MacOS has a feature called Purgeable Storage. This allows the system to clean out old files, temporary files, and files that can be re-downloaded easily when there is limited free space remaining. Apple provides some tools to leverage this, but it’s really something I’d consider to be a “Consumer” feature as many enterprise technologies don’t leverage it. The most useful measure of free disk space comes from inspecting the Macintosh HD in Finder. Also, keep in mind the purgeable storage process is fairly slow, so filling your entire drive in one go will generally not work out too well.
If you have enabled TimeMachine, then local snapshots are taken periodically and retained until they can be offloaded to an external device. Once they have been offloaded they are marked purgeable and will be cleaned up when the space is needed, but they will not be deleted until they are offloaded so this commonly bites people who set up a Time Machine volume but then never connect it again.
Too Small of an SSD
If 80GB of user data is filling your drive, then I’m fairly confident you’ve issued a 128 GB device. This would, to be polite, not be something I’d recommend in any kind of a business. With updates, applications, management tools and whatnot, the usable storage space for most users on a 128 GB drive is somewhere in the neighborhood of 20GB which is super duper tight for most users and would require lots of time and energy spent managing that space. For a home user, this is something that could be managed by using Photos and iCloud Drive and intelligent space management as those are all integrated into the operating system but with a business environment you aren’t using those tools and need to have way more space to work with.