Weird storage declaration from my computer
I would ignore it. That graph is particularly unreliable and it's unclear sometimes, even when it's showing storage that is possible within the computer, what exactly it is counting. If you want a better overview of your storage, use something like DaisyDisk.
tubedogg is likely right but I would be tempted to boot into recovery mode and run Disk Utility then repair disk and fix permissions. It might be the symptom of a disk issue that could be easily fixed.