The Tibetan calendar is based on the Kalachakra Tantra and like other tantras it is ambiguous, even encrypted, and stands in need of interpretation. The commentators on the Kalachakra Tantra which bothered to comment on the astronomy gave different interpretations, hence we have different calendars. Since Kamstang Kagyu follows the Karmapa, they are going to give a lot of weight to the Karmapa's interpretation of the Kalachakra and use his calendar.
Imagine a man who stands before a mirror; a stone strikes it, and it falls to ruin all in an instant. And the man learns that he is himself, and not the mirrored man he had believed himself to be.
-- Gene Wolfe