They could stop WEEDEN, TANNEHILL, and HASSLOLEBECK though. They have to be doing some stuff right some of the time. Really it's more like those offenses just suck and implode all too often. Rams are about the same, you're right, but the Panthers are better than those 5.
Look at points scored. Wash is in the top 10. Indy is out of the top 20 (21st
). This could be that the Colts have faced better defenses, but looking at the schedule I don't see anyone major here save Chicago.
And you do need to care who the Colts have beaten. If they had played a schedule with better offenses, they probably don't win the games they have won. Who they've played is only relevant when looking at Luck and Griffin individually. Of course, who you beat to get into the playoffs doesn't matter. That's a team thing though.
Also, look at total defense. Wash (28) is lower than Indy (19). Again though, Wash has faced superior offenses overall.