It's possible that there is some other bug effecting it, but double attack is two attacks at -4 each. Worst case maul is one at full accuracy and one at -4, and can be used every round instead of once per three rounds.
Accuracy increases by two per level - finesse, accuracy and leadership perks increase it substantially as well.
There's some faint chance at level 5 that a regular weapon that does double or more the damage of a maul one would be better in some circumstances. At level 20+ there is no hope of that whatsoever*.
* Yes yes, I know, what if you are surrounded by low level stuff that you could kill with one sweep if the weapon was higher damage but the maul weapon leaves them on 5hp, whatever 
