On a recent analysis, structural requested a breakdown of load components. I provided the operating case and the sustained case. I then tried a weight only case and a pressure only case. The weight only (and also the WNC case) yielded loads which varied from the sustained loads. No values showed up for the pressure only case. At some supports, the weight only case loads were greater, as some less. Sometimes the magnitude of difference was large, sometimes small. I told structural to use the sustained case loads. Is there a logical explanation of why these difference occurred?