It amazes me how many people are offended and have too much pride to accept being 'b list'. Unless you really think you should be everyone you knows best friend equally, I don't understand the problem? And I'm usually quite sensitive about being left out etc, but I've never expected to be invited to every event one if my friends throws (even if a close friend the event may just be aimed more at a specific group).
I've never minded evening invitations for weddings and was delighted the 2 times I was bumped to a day guest at last minute because someone dropped out! It means they ideally wanted me in the day originally but didn't have the budget / space. Can't you look at it like that? They didn't have to invite you at all, but they did when they could!
You don't say enough about the event or how the invitation was received to be clear if that's why the invitation was late. Other reasons could be genuine mistakes eg both thinking the other invited you, mistaking thinking you were on a WhatsApp group you weren't just error I number of invitations they wrote etc, none of which would offend me.
I think the only time it would stop me going is if th invitation was reluctantly given, eg they were put on the spot by a mutual friend discussing it with you present and blurted out ' would you like to come too' out of embarrassment or something.
But other than that I think many people on this thread seem to be looking for reasons to be offended.
Oh, and for the people implying there are less numbers pressures for a bbq at home than a wedding - do you live in a mansion and have a live in cook or something? I had a small wedding but would still never invite that number of people to my house for an event - too stressful and not enough space!