ivatar uses gravatar way too often?

If I make up some random hash and use it, I get:

http://libravatar-stg.fedorainfracloud.org/avatar/40f8d096a3777232204cb3f796c577b7?s=500

vs.

http://cdn.libravatar.org/avatar/40f8d096a3777232204cb3f796c577b7?s=500

In the first case, gravatar is used as a source to get avatar. In the second case, libravatar's default avatar is used. I wonder what are the rules when gravatar should be used and when not.

This is more a question than a bug report.