Cost center commitments carry forward not matching

Dear All,

I executed S_ALR_87013648 for the period 1-12, 2011. I entered the cost center group.

Now when I executed the same report again for the period 1, 2012. I am not getting the same no of cost centers as the commitments carry forward.

What could be the reason for the discrepancy? How can I identify the error?

Thanks

Rosalyn