I recently signed up for the Wikimedia-L list and selected the Digest Mode. Since then I've received an average of two to three "Digest" emails each day. Yesterday I got six! In reading the description of digest mode, I now see that it says we'll "usually" get one email per day, but "possibly more on busy lists."
I humbly submit that when users ask for a Digest, they are saying two things: 1) I don't need to follow this discussion closely and 2) I don't want to be bothered by a lot of emails. Therefore an email "digest" that comes six times a day neither meets users' needs nor conforms with the expectations users have from other services.
I'm not sure how the system decides to send out a digest. Sometimes a digest is sent in the morning that contains only two topics. Whatever the logic or the reason for the original decision, please enable those of us who want to follow events but not so immediately to get only one email per day.