Life is too short to pore over who was sent what when, and to get in a huff over these perceived slights.
Being on the B-list is not an insult and nor does it mean they don't really like you. Friendship isn't a binary "you're in or you're out" thing. I have friends I've known for over a decade but never become super-close to. They're on my B-list and I'm probably on theirs. At the same time I enjoy their company and having them somewhat in my life, and if there's a reason we never got closer it's to do with factors like geography, time limitations, slightly different leisure activities.
I've also had plenty of B- or even C-list friends who actually became extremely close once different situations threw us together several years into being acquainted. Someone being B-list now can be a reflection of circumstances, not how much I like them.
In terms of invites, there's simply a numbers limit on any sort of party you're hosting. And unless it's something formal like a wedding I wouldn't sit and plan the guestlist. Quite often when DP and I throw a party it starts off with seeing whether our closest friends are around, then gradually expanding the invites in a fairly random way.
I've also attended a wedding where I knew I was a replacement for a drop-out. I didn't feel insulted because it was the kind of friendship where I knew we were on each other's B-list. Still had a great time, was still happy for them, they were still happy to have me. It's not a big deal, it's just how friendships work.