Resource Usage view Cost Totals versus Gantt or Task Usage View Cost Totals

Hello,

I have a schedule that shows a total cost of $313K in the Gantt and Task Usage views.  If I add the cost of each resource in the Resource Usage view, I come up with a total of only $295K.  Anyone have any idea why the total of individual resource cost in the Resource Usage view would be different than the total cost in the Gantt/Task Usage View?  Not sure this has anything to do with the server or not, but we're using MS Project Pro 2010 with Project Server 2010.

Thanks