Downloaded reports as .CSV are usually not already in an optimized and ordered format. Here is a quick How To on how to fix it within a few seconds. You can find more about Reports here.
First, open the downloaded report.
Now the report will look a bit messy. Don't worry. Click on the first column "A", so the whole column is marked and go in the tab "Data" and select "Text to Columns"
Select "Delimited" and click on "Next"
Tick the Delimiter box for "Comma" and click on "Finish"
Now your report has been formatted and is easier to work with.