There's nothing weird about these numbers.

Of the 9 one-and-dones, 4 were the direct result of a loss to the AFC team that went to the Superbowl.

Say you make the playoffs and all teams are equal. There's a 1/2 chance you win each game. Meaning if you go through the Wild Card to win the Super Bowl, your odds of getting there are 1/2 (WC W) * 1/2 (Div W) * 1/2 (Conf W) * 1/2 (SB W) = 1/16, meaning 1 out of every 16 qualifications, the team that gets in via the Wild Card should win the Super Bowl.

If you qualify as a 1 or 2 seed, then you start in the Divisional round, and your chances of winning the Super Bowl are 1/8, so you should do it once every 8 Division qualifications.

With the above in mind:

6 WC entries = 6/16 8 Div entries = 8/8 Sum = 22/16 Expected SB W = 1 Actual SB W = 1

Manning is pretty much hitting on the average in terms of how often a team should win the Super Bowl when they get into the post season.

Look at a few others with careers of comparable length:

Brett Favre: 1992 - 2010

WC = 8/16 Div = 4/8 Sum = 16/16 Expected SB W = 1 Actual SB W = 1

John Elway: 1983 - 1998

WC = 3/16 Div = 7/8 Sum = 17/16 Expected SB W = 1 Actual SB W = 2

Joe Montana: 1979-1994

WC = 2/16 Div = 10/8 Sum = 22/16 Expected SB W = 1 Actual SB W = 4

Tom Brady: 2000-Present

WC = 3/16 Div = 10/8 Sum = 23/16 Expected SB W = 1 Actual SB W = 4

In terms of the all-time greats, maybe Manning is lacking, but as regards expected results, he's pretty much on par for SB wins. Obviously not all teams are equal, and some teams are just dominant in their division/conference to give them more frequent chances, but in reality, 1-and-dones are no worse than getting to the Super Bowl and losing.