Daily Digest set, but receiving MANY emails each day with subj of "Subscription Digest"

In Subscriptions > Overview >  I checked Digest Mode, which is defined as:

"Merges your notifications into a single email, sent at the interval you specify. "

Under Preferences I set the Send Interval to be Daily.

I receive a couple dozen emails each day.  They all have a subject of "Subscription Digest for..."

Each email only contains a single item, so it doesn't appear to be a "digest."

It clearly is coming more than once a day, so doesn't appear to be following the set Send Interval.

Is this a bug or am I misunderstanding the settings?