Because two aggregations does not necessarily equate one aggregation over the same set of raw data. It is quite obvious if you think of averages for instance. An average of averages does not make any sense. The articles I posted as links for you goes in to detail to explain this - you should dive into them to get an understanding.
When I've had this issue it was because there was more that one row with the same data but different values. Make sure you have a field that truly uniquely identifies each row. In my case I thought I did but didn't. One way you can test it is, in excel, do a countif on the column with the unique field. For example, if your unique field is in column A, and there are headers, the formula would be =countif(A:A,A2). If the result is 2 or more, then you have duplicate rows.
