This is not an answer of ur Query rather I m surprised to face same problem with my less than a year MBP. Found one more issue with display update from 'About my Mac > Storage tab' (color bar with space values) and Finder 'size' col.
In color bar 'Storage' tab, combination of my 'Movies' ( green color) and 'Photos' ( reddish color) = 37+20= 57 GB. Which matches in Finder with Pictures > 'Photos Library.photoslibrary' which is taking 57GB space.
But after deleting almost 19GB of videos from Photos App, total video space now became 18GB, and deleted almost 8GB of Images. So in effect total media space now became 18+12 = 30GB. But this updated space =30GB is not displaying either in Finder or in Storage tab as well. They are still showing 57GB, means Movies remains 37GB and Photos 20GB in color bar.
But when you select all in 'Photos' app, n check info u will see updated space of 30GB, i.e 18 and 12 GB respectively for movies and images. In fact shockingly my free space value reduced. Pls chk color bar screenshots taken at 2:20am (before delete) and 1:45pm ( after delete of 27Gb space ).
I emptied my trash. Dont know why such behavior? I must be missing some extra step to clear system cache kind of action. Please help. Attaching screenshots of values from all d places mentioned above.
Also note:
'Backups' taking 13 GB space! How to reduce that? Anyway I use Time Machine for external backups frequently.
El Capitan latest version ( wanted to update to Sierra after learning up spaces).
2:20am (before delete)

1:45pm ( after delete of 27Gb media files, free space value reduced !!!!!! and no change in media spaces ).

Finder shows same 57GB even after deleting 27GB media files.

But After delete, info inside 'Photos' App shows correct value.

