There was a perfect explanation of this in another post I read today, but now I can't find it.
Basically it said, Group Availability is a "binary" result - meaning, if all the members are down, the group is down...otherwise, if any member is up, then the group is up. So, if you have a group with 100 members and 99 are down with 1 up, the group is still 100% up and available. Conversely, each member is counted individually.
In my world, a much preferred method of group availability would be a "roll-up" status like other Orion options.
Hope that explains it properly, I'll continue looking for that other post.