The Last Piece Syndrome

This is probably not true at all places, but is something I've noticed in my workplace. We have a trolley that sits in the middle of the office which is mainly used as ... well, a trolley of course. However it is frequently used as a communal food table where snacks or other types of food are put on for everyone to share and enjoy. It's one of the office customs we've had that keeps morale up and helps socially- goodness knows we need all the positive things we can get. It's a different matter when it comes to potato chips, but for foods like biscuits, muffins, cupcakes, etc, whenever there's one piece left on the plate it'll remain on the plate for a very long time. It is as if it has become invisible. Why is this I wonder?