List Calculations do not calculate totals properly if negative numbers are used, and the values are being entered directly in the list view by double clicking a cell.
For example, if there are two time cards and the Monday column has the values 5, -3, the total will not be 2. It will be 1.99. If you refresh the page it fixes itself and displays 2.
Enter the values directly in the time card form, not in the list view.