Why does OAuth 2.0 REST Authorization header use "OAuth" as type rather than "Bearer"?

This is really just an "out of curiosity" question, but when using an OAuth access token to make a call to the REST API, why does it require the Authorization type to be "OAuth" rather than "Bearer"?  I bring this up because in the documentation it says: "The built in OAuth service is based on the OAuth 2.0 specification", but when I'm using Postman to test requests, if I use OAuth 2.0 as the Authorization type, set the access token and send the request, I get a 403 response because Postman sets the Authorization type to "Bearer" followed by the access token.  If I set the Authorization header manually to OAuth {access_token} the requests succeeds - which is expected based on the documentation on using access tokens to make requests.

Thanks!