I guess the issue is: whose forecasts are they? If the OBR produces forecasts and Treasury officials say, “Well, Chancellor, we have looked over the forecasts and we think they are right,” that is qualitatively different, in the public’s mind, to the Treasury producing a forecast and the OBR saying to the public, “Well, we have looked over them and we think they are right.”
While it does say that the Treasury reserves the right to disagree with the OBR, the nature of the iterative process presumably means that will never happen, because they agree before anything is published.