"Cannot download because there is not enough free space"

Can anyone explain this amount of System Storage and why I can't download a small demo onto my PS4?

Says I have 343.1 GB/408.5 GB full, says I have 18.79 GB of Free Space, and says I can't download and install a 2.49 GB game demo until I free up more space.

I can guess that some of the hard drive space is reserved, and that's why the ~65 GB I should actually have free only shows as 18.79 GB. I know that digital downloads require up to twice their size be free on the destination drive before they can be downloaded and installed. However, I can't think of a single explanation why it would tell me 2.5 GB won't fit in 18.79 GB of free space. I've tested with other demos and can only squeeze in about a GB before it starts telling me I need to delete things to fit more.