I believe, though not 100% sure, its the cost that is being calculated in Auto-Resolve. So, to weaken them, make them (generals, after all, can't be 'recruited') cheaper. And, the Auto-Resolve isn't perfect. My belief is that, with the high cost of the Chariot, it's already deadly. Now, with the bonus of the "wall defense," they're absolutely deadly in Auto-Resolve. While in truth, they're weaker because they have to fight in the limited space of the city.