The default condition of the viewer is single page mode, as seen here. 
This is so that you can click print and get your report directly to the printer, 8.5" x 11", mostly portrait.

However, this also means that theExcel export will create multiple tabs.

This is frustrating because you cant sort and manipulate that data for your needs.

SOLUTION !  Galley Mode

Generate your report and then shift to Galley Mode by clicking this button in the toolbar.

Then export your file and open in Excel with all your data on one tab !