The description of PutResponse contains the following paragraph:

"Such a response (an empty wst:PutResponse) implies that the update request was successful in its entirety (assuming no intervening mutating operations are performed). A service MAY return the current representation of the resource as the child of the wst:PutResponse element even in this case, however."

The description of CreateResponse contains the following paragraph:

"As an optimization and as a service to the requestor, the wst:CreateResponse element of the response message SHOULD be empty, other than the ResourceCreated element, if the created representation does not logically differ from the representation sent in the Create request message and there are no extension elements; that is, if the service accepted the new representation or creation instructions verbatim. Such a response indicates that the request was completely successful (assuming no intervening mutating operations are performed). A service MAY return the current representation of the resource as the second child of the wst:CreateResponse element even in this case, however."

You can sort of get the gist of what both of these paragraphs are trying to say but it's not completely clear.

General Proposal: reword for clarity