You're seeing expected and documented behavior that's tunable. By default, WebLogic "pre-pushes" up to 10 messages to each asynchronous consumer - including MDB consumers. To tune (A) configure a custom connection factory with "MessagesMaximum" set to 0 (or perhaps ?1? -- the default is 10), (B) target the CF to the same cluster as the JMS Server (or same WL Server if you're non-clustered), and (C) ensure that your MDB CF annotation or descriptor attribute references the JNDI name of the custom connection factory.
For more information see the MDB and JMS chapters of the Performance & Tuning guide in the edocs.