The A-List/B-List thing is interesting, and I think it might really depend on the group dynamics. If a friend called me up and said 'Hey, I had some people cancel for Event X, are you interested?' I wouldn't be insulted at all, and in fact have had some great experiences that way. For me, the two thoughts 'not everyone gets invited to every event, it's not insulting if you're not invited' overrides 'they didn't really want me there'. I am sure there are certain circumstances in which I'd feel uncomfortable, or like I was only being invited cause they felt they 'had' to, but the simple fact of not being on the 'first' list wouldn't do it.
This kind of sums up my feelings/opinion on the situation.
Thinking of how such a thing would play out in my own life: DH and I are both runners and socially we are part of a large group of other runners. By "large group" I mean 50 or 60 people. Within that group, it is inevitable that everyone is not equally friends with everyone else, and that smaller groups form whose members have closer ties to each other. Also, it is possible for individuals to have closest ties to the individuals in their "main" group, but also have pretty strong ties to individuals in other groups. Things tend to ebb and flow, depending on who is training for what race (and therefore who is running together), who shows up on the standard run dates (generally Wednesday evening and Sunday morning), who stays around to socialize after running, etc.
So it is not unusual that several people I know well, who are members of my "main group", might be invited to a social event hosted by someone in another group who I know pretty well but am not as close to. Because they know the host better than I do. Especially since the 50 or 60 people I mentioned above do not necessarily include spouses - so with spouses the entire group easily approaches 100 people.
DH and I don't feel slighted if we're not invited to one of these events, and we don't feel like the host doesn't want us there. We just appreciate the numbers involved. And we attend group events where the reverse is true. So if I were to hear that Kathie ("other group" member who I know well, but not as well as my main group members) is giving a party, and Susan (main group member who runs with Kathie once a week) and her DH are going, but we're not invited, my feelings aren't hurt. With the given group dynamics, it makes sense. And therefore if I were to hear from Kathie something like, "Hey, we're having a party on Saturday night, I know it's a little last minute but if you guys are free we hope you can be there", my assumption would be that they had fewer super-close friends attending and room for people from the broader running social circle. And if we were free, we would go and feel no awkwardness at all.
So that was kind of a long way of saying that it really depends on the particular group dynamics and how they are perceived/understood by everyone involved. And there can be many subtleties, especially if a given social group is large and it's just not feasible for any one person to invite absolutely everyone. If Knitterley's social group dynamics are similar to what I've described above, then I totally get why she would go to the party and not feel strange about it.