You don't always have to be obvious. Maybe you didn't noticed that all the graphics are in Helvetica? That's for consistency. It would be bad design and illegible to change each one of those circles to a font of their category, especially decorative / display (plus these categories are comprised of many different fonts, I wouldn't want to choose just one). That's why I include examples of a serif, sans serif, and decorative/display in the following graphics.

Also, UX Collective wouldn't have published my article and Medium wouldn't have curated it if they didn't believe it was a helpful, educational piece.