This is a very important question and there are at least two possible answers. The choice greatly depends on what you think of the playwright's opinion on the theology of his time.
First, if you believe that Sophocles is like Aristotle; doesn't really believe in Greek gods; then Jocasta belied the oracles in order to attack the legetimacy of the whole religious institution underlying the Greek society (a society highly concerned with relegion) i.e. the ministers and oracles do not really know what they are doing. It is all a matter of chance. This may compel any keen Greek in the audience to think of the matter. This interpretation may well seem incongrous considering how the play ended, however if we look at other plays of Sophocles, we may find ground for such assumption. For instance, Philoctetes's question "Why then, why praise we the gods when even while we praise we find them evil?" Or the fate of Ajax whose punishment far exceeds the sin he made. This make you reflect on the nature of the gods.
Another possible interpretation is to make legetimate the pain she is going to endure. By being so arrogant, she offends the gods and a punishment becomes necessary. The myth of Oedipus remains one of the strongest and most delicate myths because no one really made a mistake that requires such consequences. All Greeks know that the House of Labdacus is cursed but dramatists seem to strive to make the gods' actions legetimate.