That is weirdly true, and I've never been able to figure it out. If I'm upset, I can say "It's fine", like it's no big deal and I'm super calm, but try changing that to "It's okay" or "I'm okay" and it's so incredibly obvious that something is wrong!
I warned my husband when we first started dating that if I ever said I'm fine or that something was fine, that he should be aware that "fine" just means I'm too emotional and embarrassed to admit that I'm not okay. So far, it's worked so beautifully, I haven't even said "I'm fine". I think he's better at figuring out if I'm okay or not than I am!