Suppose you have a den with a small number of leaders (3 in this case but doesn’t matter):

And you create a calendar event, and click ‘Add Invitees’ to add people to the event:

And you click on ‘Members’ and then ‘Select All’, and all scouts in the den are added to the event. Scouts in other dens don’t show up (as expected):

But if you click on ‘Leaders’, then the entire pack leadership is shown in the list. You’d expect to see only the 3 den leaders:

If you click ‘Select All’, then the entire pack leadership receives an email for the den event, even though they aren’t part of the den.
This behavior changed sometime over the summer. Last spring when you’d go to click ‘Leaders’ for a den event, it would show only the den leaders.