Since High Sierra, or possibly earlier, macOS has been bundling fonts in /System/Library/Fonts that are 'installed, but hidden'.
That is, they are unavailable to most apps, and not shown in FontBook or the macOS font selector window.
These include Athelas, Marion, Iowan Old Style, Courier, and...
I am working with OS 10.9.5, upgraded about a month ago. Some of this may be related to the upgrade. For a while I've been very frustrated with the default font when I open MS Word. The defaults are fonts I never use. And in order to use a different font, I have to select it each time I open...