I suspect that microsoft signed a new deal after Bethesda realized how much money they could make with the exclusivity. That, or Hearthfire never was considered part of the first deal, and Beth just decided to delay the release of it for fun. They seem equally likely.
I wish the PC market had a collective bargaining chip that we could use for exclusivity. OH STEAM WHAT?
Next time, valve, weigh in for us! kktx