[Mailman-Users] Digests more than once a day ... What determines it?

[Edward Ned Harvey]

I¹m aware ³If you turn digest mode on, you'll get posts bundled together
(usually one per day but possibly more on busy lists)²

But one of my users (the CEO) just asked me ­ and forwarded messages - he
got a separate digest for each email that went to a list, at 11:33am,
11:38am, and 11:45am today.  This seems excessive, so I¹d like to know ...

Under what circumstances will a digest be created more than daily?  Is it
configurable?