I could do the same with the Steelers D (although I would never say a team had a fluke...that's a copout). I could point out how the offense turned the ball over or there were poor special teams (a former punter averaged 26yds per punt over 5 or 6 punts one of the games this year) which caused the Steelers D to be put in a poor position - literally starting with the opposing offense already in position for 3 pts and holding them to that 3pts. The fact is, the Steelers D has given up less points the last 5 games and is ranked #1 across the board for a reason. That doesn't mean the Dallas D is bad - far from it, but they haven't been as good IMO over the last 5 games.
Not to mention, the Steelers D has faced better/hotter offenses during this 5 game stretch. SEA, SF and the 49ers aren't exactly good offenses - nor is the Steelers offense for that matter (only the Steelers have broke the 300pt mark for the season and they sit at 302pts). On the flipside the Bengals are the worst team the Steelers have played in the last 5. The Patriots, Cowboys, Chargers and Ravens (who had scored the most points in the last 5 weeks) were all putting up some good points before facing the Steelers D. If needed, I'll put up the numbers that I just researched on ESPN, but I'd rather not go to all that work as I've got enough to do today.