iTunes Connect - difference between units sold and units paid out, plus Apples commision
The discrepancy is there because of several factors that you haven't accounted for, primarily:


For some countries VAT is included in "Sales" and deducted before you get your "Proceeds"
Some purchases might be refunded by Apple because of customer complaints - i.e. you will get 0% proceeds for these
The currency exchange risk is yours. It might be that the exchange rate fluctates leaving you with less than the expected proceeds when converted to USD.