Well with the Rin's tail thing, there's a part in Noragami that explains it well. It's the exact same logic applied to it.
Ah! I love Hitchhiker's Guide to the Galaxy! But I haven't read/seen it in so long I can't remember much at all ;A;
Anyways, I'm too lazy to find out how it was said word-for-word in Noragami, but it was explained kind of like this.
They were in a restaurant, and it was asked how many waitresses there were. The girl answered with two, when there were actually five.
All of them were in plain view, but only two of them stuck out, so it was perceived as two. If she has paid more attention, she would have seen the five.
Or something like that....I can't remember it's been a few months.
Basically, people need to actually be looking for it to notice it. They aren't giving any focus to it, so the miss it - their brain doesn't really register it as being actually there.
So, they see it...but....it doesn't get registered really so it's like they don't.