There are bugs in Google Drive
I discovered there were GBs of previously deleted file (not in the trash) that were counting against my total space. I will explain how to permanently delete those "ghost" files and recover the space. But first, let's exclude some of the most obvious causes for a reduced storage space.
The files in the trash are included in the calculation of the space, so empty the trash.
The used storage reported in Google Drive includes space taken up by other Google apps (Mail, Photos) and space taken up by third-party apps (WhatsApp backups are an example).
Go to https://one.google.com/storage and check the storage consumed by Google Drive, Gmail, and Google Photos. Maybe Google is right, and the space is taken up by some other application.
To Remove Hidden App Data click Settings in Google Drive, then click Manage Apps. Finally, press the button Options that appears next to each application name and select Delete hidden app data if available.
Get rid of the ghost files
If you have reached this point, I assume you have confirmed that the extra space indicated by Google Drive is unjustified. Below, you can find the steps to permanently delete some hidden files.
Back up your data.
We are going to erase the entire drive, the visible and hidden files, so it's time to back up your data.
In the search box, write: "Type: Folder" (there is a space after the colon sign).
You will see all the folders in Drive, including some ghost folders that do not show up in the app or in the computer.
Select all folder and delete them.
Empty the trash.
In the search box, write: "is: disorganized owner: me".
You should now see all of the orphaned files.
Delete all the files and empty the trash.
Click on "Storage" on the left.
You will see all the files that remain in Drive.
Scroll down to reach the last file.
Select all files and delete them.
Empty the trash.
Wait. It may take some time for Google Drive to update storage usage.
In the previous steps, if you receive an error when emptying the trash you can select all the files in the trash, then right-click and select Delete Forever.