High Sierra shows wrong disk usage for photos in information
I recently ran into this - on my 500GB SSD, my free space was only 5% where I expected more like 75%.  I found an answer in a blog that worked for me:

Apple state that they automatically remove local snapshots when disk space is low, but their definition of low is apparently only 5GB of free space remaining – not really much free working space in 2017 when you might want scratch space of 22GB for 1 hour of 4k 30FPS footage.
On older MacOS releases, it was possible to disable the local snapshot feature entirely, this doesn’t seem to be the case with High Sierra – but it does appear to be possible to force an immediate purge of local snapshots with the following command:

sudo tmutil thinLocalSnapshots / 10000000000 4

From the tmutil manpage:

thinlocalsnapshots mount_point [purge_amount] [urgency]

Thin local Time Machine snapshots for the specified volume.
When purge_amount and urgency are specified, tmutil will attempt (with urgency level 1-4) to reclaim purge_amount in bytes by thinning snapshots.
If urgency is not specified, the default urgency will be used.