Before I stick my head in the Lion's den that is P&E, figured I'd ask here.
So, we're not entirely sure, but we're betting FMIL has been either intentionally or unintentionally inviting relatives that we are not inviting, and/or telling them that we will have a B-list and they will probably get an invite as people start declining. We have no idea who we're actually going to get to on the B-list and specifcally FMIL's extended family is probably last in line since she's gotten more of her relatives in the initial invites than FFIL, my side of the family, people our age, etc.
Anyway, one of the people not invited (and who probably isn't one of the first thirty people we'd invite off of our B-list) just bought something off our registry. The not-invited person the daughter of a relative who IS invited. The (not-invited) daughter who bought us the gift is in her 30s.
What are we supposed to do? I really have no clue ...

1st year anniversary in Victoria with a killer whale topiary!