Can anyone tell me all of the reasons a Sales Analysis sales report for all customers would yield a different total than the OE Sales Report?

I am trying to create a SA report that simply shows all customers' sales amounts by month. Unfortunately when I sum it by rep, it differs from the Sales report. I have already accounted for any customers that may have changed reps mid-month.

My Excel Direct export is not always reliable, and I will verify that is not the issue.

Can anyone else recommend what else to check?

Any suggestions are appreciated.