The jump between 12 and 13 inches is not really that big however 12 to 15 is. I think apple was more targeting the spot right between the 11 and 13inch that us

The jump between 12 and 13 inches is not really that big however 12 to 15 is. I think apple was more targeting the spot right between the 11 and 13inch that used to be occupied by the 12in powerbook which had a semi-cult following and was widely regarded as one of the best laptops Apple made.