How is the “Emails sent” metric calculated?

I'm having trouble understanding the conditions for an email to be considered "sent" to a subscriber. Over the same period (Aug 17 – pres), I see 2 different subscribers have a different number of emails sent (6 vs 0), even though I send emails to all subscribers every post. Both have been subscribed since before that period. Are any conditions of the metric determined by any actions/configurations on the client side?

https://imgur.com/a/9z369MD

I'm using out of the box blog functionality, with no additional customization.

Thanks!