I'm not sure if @Sophist-UK knew the server-side details, but this does make sense to me in light of how little in the way of HTTP caching stuff the webservice supports -- caching without the force option can only save bandwidth (server-side processing is exactly the same), so it's only going to be a real benefit if your connection is slow enough that downloading web service XML is a substantial chunk of the time used (i.e., for very low bandwidth connections where downloading takes more than 1s).