Third trimester 'maintenence feed' does require higher nutrition requirements than second trimester. Of course, after she calves, her nutrition requirements are even greater.
If the producer can get more body condition on their cows before going into winter/third trimester it is much cheaper to maintain that condition as opposed to trying to build body condition when the weather is cold. Sometimes, here in the north, we can't feed enough to maintain them, so good body condition going into the winter is very important; both for the cow and the producers pocketbook.