Why does About This Mac show different values for storage?
This seems to be quite a common problem. To fix it, you need to tell Spotlight to re-index your hard drive.

There are two ways to do this, depending on how comfortable you are with the command line.


Open a Terminal window and first of all check that Spotlight indexing is enabled for Macintosh HD with the following command: sudo mdutil -i on /, then tell it to re-index Macintosh HD with sudo mdutil -E /.


or


Open System Preferences from the Apple menu and select the Spotlight preference pane. Select the Privacy tab and add the Macintosh HD drive into the tab. This will remove the existing index. Then remove the drive and close System Preferences after which Spotlight should re-index the drive.


Re-indexing may take a while but once it is complete the figures should match.

A handful of sources that talk about this solution:

https://www.macissues.com/2014/05/27/fix-about-this-mac-not-showing-proper-disk-space/
https://www.cnet.com/news/what-to-do-when-about-this-mac-lies-to-you/
https://www.maketecheasier.com/fix-wrong-hard-drive-data-usage-calculation-osx/
http://www.macworld.com/article/2997252/storage/the-case-of-the-missing-el-capitan-hard-disk-space.html