Halliday & Matthiessen (2014: 688-9):
There is, in fact, a wide range of variants for the expression of modality in the clause, and some of these take the form of a clause nexus. This range is comparable to what we found with other types of modal assessment. If we limit ourselves first to the meaning of ‘probability’, the principal categories are as shown in Table 10-7; …
What happens is that, in order to state explicitly that the probability is subjective, or alternatively, at the other end, to claim explicitly that the probability is objective, the speaker construes the proposition as a projection and encodes the subjectivity (I think), or the objectivity (it is likely), in a projecting clause. (There are other forms intermediate between the explicit and implicit: subjective in my opinion, objective in all probability, where the modality is expressed as a prepositional phrase, which is a kind of halfway house between clausal and non-clausal status.)