rbd export with export-format 2 exports all snapshots?

Hi, Say, source image has snapshot s1, s2 and s3. I expect "export" behaves the same as "deep cp", when specify a snapshot, with "--export-format 2", only the specified snapshot and all snapshots earlier than that will be exported. What I see is that, no matter which snapshot I specify, "export" with "--export-format 2" always exports the whole image with all snapshots. Is this expected? Could anyone help to clarify? Thanks! Tony