the reason for this is that the Request class internally changes the transport layer to iframe transport whenever you set a form field (= using the 'setFormField' method).
setFormField(String vId, String vValue)
Add a form field to the POST request.
NOTE: Adding any programatic form fields using this method will switch the Transport implementation to IframeTransport.
NOTE: Use of these programatic form fields disallow use of synchronous requests and cross-domain requests. Be sure that you do not need those features when setting these programatic form fields.
Anyway, it's way better to use the "qx.io.request.Xhr" class. That's the new one: it works better and is very-well tested. With this class you can use the "requestData" property to send data to the server.