'Low Disk Space' (and Disk Usage Analyzer) vs Disks utility [duplicate]

I have posted a very similar question here before.

A similar thing happened this morning. I received a Low Disk Space warning

The volume "InternalHDD" has only 873.8 MB disk space remaining

I clicked the button "Examine...", and up comes Disk Usage Analyzer which (unfortunately) says

Could not scan folder "/mnt/InternalHDD" or some of the folders it contains

So I start the Disks utility to have a look, and the Disks utility reports that my disk "InternalHDD" has 52 GB free.

There's no immediate problem, because I was able to free up 256GB immediately on my "InternalHDD".

What I want to know is:

• Why does this happen?
• What is being measured to give the 873.8 MB free disk space number?
• What is being measured to give the 52 GB free disk space number?
• Why are these numbers different, when they are supposedly for the exact same disk at the same moment?

Here are screenshots of each of these: