I read a sentence as follows from a book by a Harvard professor(born in the U.S.A.):
Despite the fact that farming required more strenuous physical exertion than most urban jobs, the rural grain ration was set lower than in the cities.
I wonder if the last clause is grammatical since it is an adverbial modifier that follows
lower than. I have never read such a combination before, and I thought
lower than that in the cities seems more natural-sounding.