Sorting problem

When I do a sort by contract # (Column A) and I do a sum total (for column N) at the bottom of the worksheet I get a different amount then when it is unsorted. It is not a big difference in total amount but still it is not showing the exact total amount for sorted and unsorted. I need to have the worksheet sorted by contract number. Which is the correct total amount and what can I do to make the sorted and unsorted list showing the same total amount? Thanks.