I think there is continual refinement of costing. I'm not sure about that particular bill. It could be that there were changes introduced in the policy and the approach and that may have affected the costing or re-costing when the bill was reintroduced. I presume there would have been some changes from the initial bill to the bill that was reintroduced and that would have affected the costs.
On March 16th, 2011. See this statement in context.