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.
I do use maul when the force is really high to get rid of some, but in a 10 min parse those situations occur maybe 3 times. Most of the time deathfield is coming off c/d or I have to refresh shock, etc. Those mauls also do not hit as hard as the duplicity mauls either so the dps increase is very negligible.
It is also just a personal preference thing. I would just tell Xethis to stick with mad maul if it works out for him, it is fun and great for pve. As long as bosses aren't hitting enrage, it is not really that necessary to worry about the extra dps potential. I will be running mad maul for pve for a bit because I really like this build.