Halliday & Matthiessen (1999: 125):
On the one hand, a sequence may extend beyond a single clause complex […]. That is, the general potential is simultaneously semantic and grammatical; but in the creation of […] text, this potential may be taken up semantically to create a sequence that is more extensive than the clause complexes realising it. […]
In other words, once sequence and clause complex have become partly dissociated so that one sequence does not automatically imply one clause complex, the decision how to associate them in realisation becomes a meaningful, significant choice. (This significance may vary from one register to another, but the principle that the choice is meaningful is quite general.)