In your webserver log, you will always see the ASA internal IP accessing the webserver (instead of the client).
This behaviour is by design.
What happens here is, when the WebVPN user click on the link, ASA itself will fetch the data from the webserver. The WebVPN client will never have a chance to know where is this server, nor the server know where is this 'real' client.
So if you are talking about traceability, you have to do this at both the webserver logs and the WebVPN logs. Ensure the time is in sync so that you can make the correct references.