Halliday And Matthiessen (2014: 686):
The range of assessments assigned to propositions within the domain of the clause and the range of assessments assigned to things within the domain of the nominal group are not, of course, the same. They overlap; but there are kinds of assessment specific to the realm of propositions, just as there are other kinds specific to the realm of things. The common foundation is that they are both projections of the speaker’s assessment. This explains why clausal assessment can in fact be transformed into nominal assessment, as in disappointingly they forecast that ... => the disappointing forecast.