First of all, stop using that site, we have the information right here in the Challenger-in-the-Press section from real, official sources.

Now I'd be happy to explain that. The gearing of the transmission for the manual can compensate for the MDS on the highway driving by using taller gearing as the automatic's overdrive is pretty short and makes it run at a higher RPM while cruising makes it use more fuel as opposed to the manual having a taller overdrive top overdrive to cut back on the RPM level and burn less fuel. The issue with it using more fuel in the city is most likely explained by the fact that when these tests are done, they are being driven with points of cruising and deceleration when the engine does not need full power and has the MDS engaged. Also some acceleration can be done while in MDS mode if it is done conservatively, which will save fuel. Also at moments of idling, it won't burn as much fuel if only 4 cylinders are engages as opposed to 8. That is why having the MDS, it will save fuel in the city.

Does this help?