Oh, and I should add that purge is not capable of deleting all iterations of a particular revision. It will always leave at least one iteration at each revision. You cannot purge earlier revisions completely.
It helps to remember that "purge" moves from the past to the present (but always leaving the latest at each rev) and "delete" moves from the present to the past.
For instance:
A.0, A.1, A.2
B.1, B.2, B.3
C.1, C.2, C.3
Purge will can remove A.0, A.1, B.1, B.2, C.1, and C.2, but it cannot remove A.2, B.3, or C.3
Delete can remove any of them, but only in the order they were created. C.2 cannot be deleted unless C.3 is deleted first. Likewise, nothing in rev B can be deleted until all of C as been deleted.