Neo4j has control end to end. Cayley storage is handled by other databases. You can say one is a graph database and the other is a graph library on top of another database. Same as Titan now Janus.

Think about Datomic. It can use Riak as a backend or something else but it is still seen ad Datomic to the outside world. (You can also compare it to Lucene vs Solr)

Wait, Datomic with Riak as the backend? That seems a ridiculous level of abstraction, since Riak itself uses pluggable backend stores (Bitcask or LevelDB). Like creating a programming language whose interpreter runs on the JVM.