Jack Duluoz narrates the events of the novel in retrospect: he knows what's going to happen at the novel's conclusion, and he guides us through the events with this end in mind. That's why we can point out "signposts" or direct the reader to pay attention to certain details. (At one point Jack writes, "I was healthy and feeling good, [yet] I would be going mad in this canyon in six weeks on the fullmoon night of 3 September" [4.1].)
Yet there is an immediacy to his perspective that keeps the writing reflective of Duluoz's current mood. Just look at Jack's wild language as detailed in the climactic Chapter Thirty-Seven. These are not the calm and careful words of a man recalling with measured perspective what happened to him years back. These are instead the frantic words of a man going insane. Similarly, Jack-the-writer is still consumed with the same confusion that plagues Jack-the-character; as he writes time and time again, "I dont understand what happened at Big Sur, even now" (37.4). Jack's inability to understand the events at Big Sur keeps the novel in the same mood and tone that characterize the events that occurred.