503 quota exceeded

[mp]

I'm seeing an issue where I get 503 (quota exceeded) errors when making API calls to fetch user information.  The code is running on App Engine.  When I run the code on one app id, I do not see any 503 errors, but when I run the same code on the same dataset on a different app id, I get lots of 503 errors.  What could be causing this?

By the way, I'm aware that Google recommends exponential backoff in this situation:

https://developers.google.com/google-apps/documents-list/#handling_api_errors