After an email send, the number of emails sent is much lower than the number of people that qualified for the email.
One possible reason you are seeing a different number of emails sent versus the number of people that qualified for that is email is because of communication limits. If a person qualifies for an email, Marketo will try to send it. However, if a lead has already reached their daily or weekly communication limit, Marketo will not send the email (with the exemption of operational emails).
To check to see if a lead did not get an email due to communication limits, follow the steps at Email Program Fails to Send Emails.