Why Can I See an X Reply on a Profile but Not in the Conversation?

I’ve noticed that some replies on X appear when I open someone’s profile, but I can’t find them in the conversation under the original post. Is this usually because of hidden replies on X, a visibility setting, or the way the replies are being sorted?

I’m curious whether the reply is actually unavailable to some viewers or simply harder to find in the thread. Does the behavior change depending on who posted it or whether I’m logged in?