How to find the cause of variance between invoice total and the number on income statement ?


I run a net sales report from AR explore=>customer history invoices view. I use the invoice total minus sales tax, which gives me the net sales for that period. However, this number does not tie my income statement number. My June net sales total is off by $7.45 and May is off by $35.00. Can anyone give me some clue on how to find out the cause for this discrepancy ??? 

Highly appreciated,