Fiction is a higher art than non-fiction. Martin is generally credited with being the forerunner of the non-fiction novel that Truman Capote and Norman Mailer popularized in books such as Capote's In Cold Blood. Martin, for all his dedication to getting the facts and telling the story faithfully, realized that objectivity had its limits. The writer could not hold up a mirror to individuals in a society and refine himself out of existence. A writer has to be selective.
Complete objectivity is impossible.
He'll pick the facts as he sees them and write them in light of his own experience. That's really all he can do. Martin certainly gave himself lots of space for selectivity. He was as thorough in his writing as he was in his research. His notes for one magazine article ran to 3,000 pages.