The differences in peak power can be accounted for in multiple ways, but primarily it is because the engineers have tuned each engine to produce a particular power range and torque curve.
The tune on the Ram probably maximizes low-end torque, while the hp probably falls down alot sooner after peak, where as the Jeep probably has the curve pushed a bit more into the middle letting the hp run out a few hundred rpms more and the LX tune probably goes even further into the RPMs.
Peak hp/tq don't mean nearly as much as how much total power and torque you have under the curves and where its placed in comparison to your transmissions gearing...