I came to understand OAuth2 much better when I realized that it exists to make the lives of big companies easier, and to make the lives of small developers possible. If BigCo only offers an OAuth2 API, then developers will figure it out because they have no choice. And from the point of view of big companies, what matters is that they implement something that meets their needs, which they can pretend is a standard.

Ambiguities give big companies the freedom to do the different things that they want to do while everyone claims, "We're following the standard!"