Those Colts were 12-4 and the #1 seed in their conference, with a +67 differential. They are similar to the Giants in that they were much improved defensively in the playoffs (in fact, Indy finished with the #1 scoring D the following year, 2007). The '90 Giants were 13-3 and had the #1 scoring D.Peyton Manning's Colts were worse than the Giants. The Giants won the NFCCG b/c their defense dominated every drive in the second half and OT. Alex Smith looked like a high schooler in that game and the Giants capitalized on mistakes. The 1991 Giants were probably worse, too. Now that was an upset.
The '01 Pats deserve mention here; they were only the second five-loss champ, and they were 19th in yards offensively and 24th defensively. All the six-loss champs are since: '07 Giants, '10 Packers, '11 Giants (the only seven-loss champ), '12 Ravens.