In addition to what azrsx said, you have to look at how many teams were in the league back then. When the Packers won it all in 61, 62, and 65, there were only 14 teams in the NFL. Even with winning the first two Super Bowls in 66 ad 67, there were only nine teams in the AFL, meaning only 24 teams in the league (the Falcons became an NFL team in 1966). Also, the playoffs were vastly different. There was no playoff system until 1967, so the first four championships came under the old format, which was just a playoff game between the winners of the Eastern and Western division of the NFL. Not the same kind of opportunities for upsets like there are today.