Job stats will be different than the Class stats. Perhaps Bard has less attack than archer and the other DDs will most likely have more attack than their Class.

The increased abilities of WHM would mean that you are meant to rely on them for support since they will be able to hold the lion's share of it, with bard for backup, so the DDs will be able to focus on DD'ing. It's the whole point of the Job system to specialize while boosting the character's abilities.