[Exim] Defer logic

In sending out BSMTP mail to the same list of users, on one day, exim did a
defer=-42 (or some such) and completion of the delivery took about 20
minutes, while yesterday, a longer piece to the same distribution list set a
defer=-1 and completed in about 1 minutes

What is the logic/mechanism that determines how quickly exim completes
delivery?

David