Brennan is standing below on the street, hears gun shots, does the logical thing and looks for the source of the noise and sees Oswald with his rifle.
Except he didn't, because in the Z-film he can be seen watching the motorcade at the time the shots rang out and the shooter was just about completely hidden behind the wall next to the SN window.
This doesn't seem like a great mystery.
What is a complete mystery is how a reasonable person can take anything Brennan said seriously.
What Brennan swore to in his affidavit on 1/22/63 is the truth as he remembered seeing the events at the time of the murder..... He was not lying...... but he may have been confused about some aspects. It was only after seeing Lee Oswald murdered
( who he knew was innocent of shooting JFK) that he saw the light and decided to support the official tale.