Can an employer refuse to offer a severance package if they have made exceptions for others?

I accepted an offer of employment instead of severance in a closely related department. I accepted the offer based on the job description and reporting structure provided at that time by the "new" manager. Now that I have moved to the new group, my manager has now informed me that my responsibilities have inceased by 50% and the reporting structure has changed. I was informed that this was the "original job" but those details were never communicated to me. This is not what I accepted and the new manager never informed me of the major changes in the job. I have asked to take the severance package since this change represents a 50% increase in workload; however, they are telling me that the severance is no longer available because I accepted the offer. My company does have a written severance policy; however, exceptions to the rules have been made in situations like mine.