All Casts vs. Last Cast

[Anonymous]

The engagement program I'm referencing is set on a 14 day drip. Each recipient that receives an email is automatically sent the next email in the stream on the next cast. Can anyone explain (clearly) to me why I see such a variance in all casts to last cast? It is my understanding now that the drop either means people unsubscribed or the data was incorrect (i,e bad email addresses).

[Josh_Hill13]

All Casts - total emails sent all time for this Email whenever it is Cast.

Last Cast - # emails sent for this email at the last cast.

This is a rolling number. Each time a new person enters the Stream, they get Email 1 at the next Cast Date, then they move on. So the # will be different each time.