You generally don't need to, because as the details say "This value will fluctuate according to system needs."
My guess is that you have a lot of files in your iCloud drive or Photos is set to "Optimize iPhone Storage". These will download a copy of the files when you are using them, and delete them after they are not used for a period of time or when needed to free up space.
If you really need to free up the space, add a large file to your phone (which will trigger freeing of cached data), then delete it.