One example when the narration shifts from third person limited to more of an omniscient narrator is near the end of the story:
THIS STORY is not his biography, remember, although things creep into it which have nothing to do with those dreams he had when he was young. We are almost done with them and with him now. There is only one more incident to be related here, and it happens seven years farther on.
You can see how the voice shifts here. This isn't an account of Dexter's actions or feelings; instead, the narrator steps outside the story for a moment to speak directly to you, the reader. This serves a couple of purposes. First, it distances the reader from the author a bit more, helping to suspend disbelief of this fictitious story through a narrator who seems to be an all-knowing and impartial reporter of the events described. It wraps the reader into the story, shifting to a first-person usage of "we" as the story reaches its conclusion.
This segment follows the period when Judy pulls Dexter away from his fiancee, showing up with another sad tale just long enough to end his prospects of happiness by asserting her emotional control over him once again. In the end, Dexter does not regret spending that month with Judy and losing Irene in the process.
So this little slip in narration reminds the reader that significant time has passed. He loved Judy for ten years prior to this month he spent with her, and now seven additional years have passed when the narration brings us up to speed. The narrator is almost "done" with Dexter, which is a bit dismissive and sets up his eventual and complete loss of Judy and the life he's always envisioned. Yet the narrator also notes here that this story is not Dexter's biography; this seems to imply that Dexter recovers from this loss and goes on with his life. Life doesn't end for Dexter when he realizes that he has to leave behind the "illusion, of youth, of the richness of life, where his winter dreams had flourished."
The narration allows readers to understand that there is eventually more to Dexter and to his life's story than the eternal longing for Judy Jones.