Historically, regular season point differentials predicted the likelihood of reaching the Super Bowl very well. In 2010, Green Bay may have been a "Wild Card" entrant based on their record, but their aggregate differential of +148 put them on top of both conferences. Since 2002, however, regular season differential have become less meaningful. The NY Giants' regular season aggregate point differential of -6 ranked 10th out of the 16 NFC teams -- Arizona's ranking in 2008. Since the 2003 season, four NFL teams whose point differentials placed them 7th or lower in their conference reached the Super Bowl with another 2 with rankings of 5.
Prior to 2003, the Raiders held the award for the lowest ranked team to reach the Super Bowl in terms of point differential at 6th in the AFC in 1979. From 1978 to 2002, four teams ranked 4th made the Super Bowl. On the flip side, of the 50 teams making the Super Bowl in the time frame, 40 (80 percent) ranked either 1 or 2 in their conference in terms of point differential. Since 2002, 50 percent have ranked 1 or 2. Some of the Super Bowl winners with eye-popping regular season differentials have been '85 Bears (+258), '84 49ers (+248), '99 Rams (+284), and '96 Packers (+246). The 2003-present era contains the '07 Patriots with a whopping +315. (Stats from Pro Football Reference).
What's driving this shift? I have read suggestions of greater parity but that merely restates the question in a different form. The most obvious change has been the playoff format. In 1978, the league introduced Wild Card" playoff format in 1978 and adjusted it in 1990 and 2002, suggesting a possible explanation. This maybe the answer, but it's not simple even with it. The top two teams have received Wild Card "byes" under under all three WC systems (1978 2 Wild Card play each other, 1990 -- three Wild Card teams + 1 of three division winners, 2002 -- two Wild Card teams and two of four division winners). The format change and league divisional restructurings have increased the number of teams in the playoffs, providing the opportunity for lower point differential teams to play whether they enter as a WC team or a divisional winner. This may increase the likelihood of a team with a marginal overall regular season but jelling at the end of the season reaching the playoffs and disturbing the outcomes.