Other mistake: When Jack asks Greg to throw a brick at the RV window and it hits the car windshield Dina and Pam pop their heads out of the RV door looking shocked. In the next scene Pam and Greg are walking into the RV like it's the first time, seems like Pam wouldn't have that reaction to the RV if she's already seen it.
Answer: He writes about what happened immediately before and after the blackouts, and when he flashes back, his mind fills in the gaps and allows him to manipulate the events.