Difference in formation of P&L and Sales bu Item Report

Difference in formation of P&L and Sales bu Item Report

Hello!
I'd like to ask you a question. I've downloaded a P&L report and checked sales by product and accounts of product. My colleague  also wanted to check it, but he was not able to download P&L reports, so he downloaded Sales by Item report. Although we assumed that it should be the same ( because there are the same items and it's accounts)We've found that there is a difference in these reports regarding total amount. 
Could you please tell me why there is a difference in these reports.
Best regards,
Zlata Malysheva