Discrepancy between 'Was sent email' and Email Performance report

I'm doing some reporting on volumes of emails sent last month and was cross checking a smartlist of 'Was Sent Email XX' between '1 Oct and 31 Oct' against my email perfomance report with the same setup. However my smartlist says that around 7000 less emails were sent than are recorded in the report.

Has anyone come across this before or have any ideas what might be happening?

Thanks