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.)