The number of results reported is just a half-assed guess. The server which got the search term calculates it by looking at what fraction of documents matched, what is the total index size, etc.
Usually, the search phrase is hashed and the phrase sent to a backend server based on the hash. So 'oauth authorization' and 'oauth "authorization"' hash differently and get sent to different backend servers. These two calculate the 'number of results' figure differently, and hence you get the difference.
That is my guess at how you're seeing these numbers; I don't work for Google (but have some knowledge of another search engine).