If you do a tag search to find out how many contacts are in the tag group this could be for example 800 contacts.  

However it might be that not all contacts in the tag group will have an email address - this could reduce the number of contacts in the tag group with an email address down from 800 to lower figure - for example 750 contacts in the tag group may have an email address. 

If these contacts have been emailed before then this can reduce down the number of emails sent out to this tag group. 

For example, if any contacts have previously unsubscribed from receiving email from you or the recipients email address has previously marked the email you sent in the past as spam or if the email were blocked then the email would fail to be sent - meaning that this further reduces the number of emails actually sent via the e-Marketing system. 

Please click here to read more about understanding the results of your e-Marketing campaign report.