"Last emailed" longer than expected

Under what circumstance would the “Last eMailed” column for a particular user be quite some time (11 days in an example I just noticed), even though they haven’t visited since then and their profile is set to send digest eMails daily?

If they haven’t been back in 11 days, why are they are they not receiving eMails every day, given their setting for eMails? Does the “Last eMailed” column in any way reflect actually deliverability i.e., if the eMail bounced?

A little more context on this. See the highlighted contacts. They are all valid members, with good eMail addresses, etc and their profiles are set to send digest eMails daily. But it seems like nothing is being sent, of the “Last eMailed” column is accurate. Any explanation for this?