Why do system information and du show drastically different numbers
Cause:
The &quot;thin&quot; command from tmutil thinlocalsnapshots [sometimes] does not delete all data content.

When this happens, after thinning the localsnapshots you may notice that not all free space were recovered [this is the OP problem he asked for solution]
The only working solution to recover space is to manually delete all localsnapshots.
After thinning you must verify if the thin command was successful by executing both commands below:
tmutil listlocalsnapshots / and tmutil listlocalsnapshotdates
If their return is EMPTY, then everything was deleted.
Otherwise if they return something, then the thinning process did not recovered all free space. (see image below for such situation where I repeatdly run the Thin command and the List command)

On the above image, everytime the command tmutil listlocalsnapshotdates is run, there is a list output. (so there are data residues left)
Snapshot dates for all disks:
2019-08-28-082639
2019-08-29-113651
2019-08-29-135729
2019-08-29-145723
2019-08-29-155709
2019-08-29-181243


Solution:
It is necessary to manually delete them all:
tmutil deletelocalsnapshots &lt;snapshot&gt;
One by one:
tmutil deletelocalsnapshots 2019-08-28-082639
tmutil deletelocalsnapshots 2019-08-29-113651
tmutil deletelocalsnapshots 2019-08-29-135729
tmutil deletelocalsnapshots 2019-08-29-145723
tmutil deletelocalsnapshots 2019-08-29-155709
tmutil deletelocalsnapshots 2019-08-29-181243


After doing, check again:
tmutil listlocalsnapshotdates
Snapshot dates for all disks:


There is no listing result as output. So now you have freed all possible space!
The OP got 152GB of free space by manually deleting all localsnapshots, which was not recovered by the thinning process.
This is very, very common on MacOS Catalina, I can assure it, and Apple is aware of it and changing things during the Beta phase. But it can also happen on Mojave and/or other versions.