The reason is simple: The absence of Ben Roethlisberger in the first four games of the season.
With Big Ben suspended, the Steelers relied on their running game and their defense, and that formula was going enough to produce a 3-1 record that included victories over playoff contenders Atlanta and Tennessee.
Dennis Dixon and Charlie Batch started the first four games at quarterback and more often than not struggled, but now Roethlisberger is back and that brings another dimension to the Pittsburgh offense.
In particular, it makes wide receiver Mike Wallace a guy to watch again. He's one of the fastest players in the league and is averaging a whopping 25.1 yards on his 12 catches this season.
Roethlisberger will look to him at least a couple of times in Sunday's game, so the Dolphins secondary has to be aware of him.
Underneath, the Steelers have veteran Hines Ward, who not only is one of the toughest receivers of his generation but also is a tremendous short- to intermediate-range targets.
Tight end Heath Miller also is a good receiver, and Dolphins fans remember him burning Miami for an 87-yard touchdown in the 2006 season opener.
Mendenhall has 495 yards in Pittsburgh's first five games, and his 50-yard touchdown in overtime helped the Steelers beat Atlanta in the season opener.
He's a between-the-tackles runner with the ability to break tackles and also is nifty enough to get around the corner.
The offensive line, while probably not up to Steelers standards, still is a solid unit.
When tackle Willie Colon was lost to a season-ending injury, the Steelers went out and added veteran Flozell Adams, the longtime Dallas Cowboys left tackle.
He's lining up at right tackle for Pittsburgh, with Max Starks on the left side. Former University of Florida star Maurkice Pouncey has made an immediate impact, winning the starting center job in training camp.
The Pittsburgh offense, though, starts with Roethlisberger, who cemented his reputation as a big-game player in the Super Bowl a couple of years ago when he rallied the Steelers against Karlos Dansby's Arizona Cardinals.
In addition to being accurate and having a strong arm, Roethlisberger also is a huge quarterback with a knack for shaking off would-be sacks and keeping a play alive.
Therefore, it will be imperative for Dolphins defenders to tackle well and not let up at all until the whistle has blown.
The bottom line is the Pittsburgh offense might not be quite as good as the Steelers' dominating defense, but it's still a very good unit — one good enough to help the Steelers make it to another Super Bowl this season.