Aristotle on Plato's Republic

I was just reading Aristotle’s Politics and am amazed how he completely misinterprets his teacher; Republic was not about city construction, but about trying to argue that the just life is better then the injust life. Surely, a man of this kind of calibre could not have so easily misunderstood Plato’s work. Maybe he wanted to make a name for himself? In that case, what kind of philosopher is he if he writes down his misinterpretation, which are on purpose, in his books?