One reason could be that the parsers don't make use of that extra force by spamming maul but otherwise that's all I can come up with. Maybe you aren't using the madness rotation properly?
I do know the Madness spec well, I ran it all through 1.2 and 1.3. I did mention earlier that maybe I am just a bit better with Mad Maul because there is one less dot to track. IDK though cause like its being refreshed right along side my Discharge DOT so I don't really see how you can be good at one spec and not the other. I am pretty sure its just a scaling thing. A few months ago Thrash was king and it was worth it to not use Shock in your rotation, now Shock is back. I think different abilities scale differently and your main stat vrs power affects abilities differently. I am not in a hard core raiding guild so my gear on my Assassin is not as good as most of you, I'm only in Black Hole (augmented) and currently in the process of upgrading Armorings. BIS Assassins just have better return on their force abilities as I currently have a better return on my Thrash/Maul.
Either way the spec is fun to play. Most DOT specs tend to see lots and lots of little numbers streaming over their targets, Mad Maul is fun because about every 10 sec you get to see some big crits via Maul. Even for those that do Parse higher with Madness can still take Mad Maul to their Operations and still be very viable and competitive with other dps specs.