How can a fb friend I filtered from several posts know about them?

A fb friend has informed me that she noticed I made several posts to my timeline but that she apparently was filtered so that she could not see them. She was correct. I do custom who can see my posts as does just about everyone else who uses facebook. The friend is very pissed that I'm omitting her from the several posts and it's causing an issue for me that I don't feel I have to deal with. After all, I have every right to limit who sees certain posts. That's the essence of privacy settings!

Putting aside the fact that such a complaint is way off base, my question is how the heck can she see from her fb page or computer the fact that I'm posting things. She even tells me the number of posts I've made (e.g., 11 or 8) that day that she couldn't see. I've asked her to tell me just how she can know that information but she refuses to reveal how she does it.

Doesn't the privacy setting feature mean that those filtered out can't even know about the post, let alone not see the content?? So, please, can someone tell me how she can know this? Is there something on fb that can reveal this? Is there another site or app that can provide this information? I've asked this on the Facebook help community but, as usual, not one response.

I understand that I can just unfriend this rather nosy and demanding friend so I'm not looking for that advice.

I really look forward to someone telling me how the heck she knows?