This behavior of InnoDB is absolutely fine and nothing to worry about. As a fork of InnoDB xtradb uses most likely the auto_increment handling algorithm than InnoDB. Its designed up from (not exactly sure) 5.1.x to allow higher concurrency while doing certain operations like INSERT ON DUPLICATE KEY UPDATE
operations in InnoDB.
You will find more information on this topic in the docs: AUTO_INCREMENT Handling in InnoDB