You don’t know their circle
You don’t really know the other people this friend seems to hang out with. You have heard a lot about her other good friends, but they’re all just stories to you. You’ve never met the people she clearly spends a lot of time with. When you catch up, you hear about the trips or girls’ nights she had with other people. You aren’t a part of those memories. You just get the play by play after the fact. Likewise, this friend has never met some of the most important people in your life. It says a lot if introducing one another to your circles is not a priority.