The Google API Console distinguishes between Google+ API and Google+ API (Sign-in). What's the precise distinction here, i.e., what specific calls does the latter refer to?
(The sign-in quota is showing up as many per second per user, which would seem excessive if it really is just a single user signing in. I guess it's an arbitrary value.)
The methods that require authorization of the https://www.googleapis.com/auth/plus.login scope fall into the "Google+ API (Sign-In)" quota. All other Google+ methods fall under the "Google+ API" quota.
"Google+ API ("Sign-In") quota covers these methods:
Google+ API quota: - All other methods