Why is the receipient count in CiviMail less than the number in the group? No, I mean, specifically!
Here's a piece of sql that answers the question to some degree:
select count(distinct(gc.contact_id)), gc.status, e.on_hold, c.do_not_email, c.is_opt_out, c.is_deceased from civicrm_group_contact gc inner join civicrm_email e on gc.contact_id = e.contact_id inner join civicrm_contact c on gc.contact_id = c.id where group_id = the_group_id_you_care_about and e.is_primary = 1 group by status, on_hold, do_not_email, is_opt_out, is_deceased;

And here's what I got from it:
+--------------------------------+---------+---------+--------------+------------+-------------+
| count(distinct(gc.contact_id)) | status  | on_hold | do_not_email | is_opt_out | is_deceased |
+--------------------------------+---------+---------+--------------+------------+-------------+
|                          10374 | Added   |       0 |            0 |          0 |           0 |
|                           1085 | Added   |       0 |            0 |          1 |           0 |
|                              9 | Added   |       0 |            0 |          1 |           1 |
|                              2 | Added   |       0 |            1 |          0 |           0 |
|                              2 | Added   |       0 |            1 |          1 |           0 |
|                           3792 | Added   |       1 |            0 |          0 |           0 |
|                              1 | Added   |       1 |            0 |          1 |           0 |
|                              3 | Added   |       1 |            0 |          1 |           1 |
|                              1 | Added   |       2 |            0 |          0 |           0 |
|                              1 | Pending |       0 |            0 |          0 |           0 |
|                              2 | Pending |       1 |            0 |          0 |           0 |
|                           2546 | Removed |       0 |            0 |          0 |           0 |
|                            313 | Removed |       0 |            0 |          1 |           0 |
|                              2 | Removed |       0 |            1 |          0 |           0 |
|                              2 | Removed |       0 |            1 |          1 |           0 |
|                             67 | Removed |       1 |            0 |          0 |           0 |
|                              1 | Removed |       1 |            1 |          1 |           0 |
+--------------------------------+---------+---------+--------------+------------+-------------+

The pending and removed could be ignored, but it's also nice to see how many people removed themselves from your list.
Note this doesn't give a complete answer, there's still about 800 to go to get down to 9560 receipients, but in this case, it was due to 800 duplicate email addresses. I'd probably need to use a separate sql to show that, but patches welcomed.
Also seems like a civireport to do this might be nice.