Note: groups.io will be down for maintenance this evening, starting at 11pm Pacific Time (6am Tuesday 8/3/2020 UTC), for up to two hours.
Which sort of comes back to Chris' point that the notifications workVive la différence!
The underlying mechanisms are in fact different in a relevant way.
The email notifications stack up in my Inbox all day long until I get around to looking; hence the reaction people had in the first day about they and their members being "bombarded", "flooded", and other descriptions of an undesirable excess of notifications.
The web notifications are transient: here and gone on my screen in a few seconds. They don't need combining because they don't build up anywhere. I'm not sure how the app notifications will be, hopefully they won't stack up in the notification bar.
That said, I think the combiner could be modified to have an immediate "first out" if that were desirable. I had advocated such a thing, but I'm no longer so concerned about it. It would preserve immediacy in a low-activity group, but it might add to the overall complexity and apparent inconsistency of Groups.io behavior.