Meetup: Many members, few attendees

I started to collect this information a while ago, but have not got too far. In any case I have some intersting (or strange) findings. I foudn a number of very big groups with 10-40,000 members that have very small meetings. I wonder how does that happen? Many people sign up to the group but then very few can actually attend, but they are still in tbe group.

Do they still get notifications for every event that they then ignore?