And I've double checked the individual's Email Delivery setting has "NO EMAIL" checked.
Is it possible the individual has two subscriptions?
You could go to the group's Members list and click on the Delivery column heading to sort by Delivery. You may need to click twice to flip the sort to reverse order, that will bring the "Summary" subscriptions to the top. If there are relatively few Summary subscriptions you might be able to spot an alternate subscription by the same individual, if his/her two email addresses are similar.
?
I will try experimenting and changing his Email Delivery setting to Daily Summary for a day and then back to NO EMAIL to see if toggling the setting fixes it.
If it does, that fact should be reported to [email protected] - that would be a bug.
If that doesn't work and no one else is seeing this, I'll be at a loss.
I'd try to get the individual to forward me a copy of such a Summary email, with the full header included (if that's practical in his/her email interface); or examine the header him/herself. Near the top of the header is an X-Original-To: field that shows the address to which it was sent. The ordinary To: field might give the answer, but might instead have been modified if he/she has one email address forwarding messages to another.