How can last cast total be significantly higher than number of leads in the stream?

On the Streams Leads view I can see that the total number for "Last cast" for each stream is significantly higher than the number of leads in that stream, and also significantly higher than the sum of the "Last cast" numbers for each individual email in the stream; deactivated content included. (Total Last Cast 704000 vs Leads in stream 49000).

How can this be? I'm using a lot of programs in my streams but they all just send one email!