May contain traces of milk?

On the ingredient list of this food product it says may contain traces of milk at the very end. does this mean like "oh we mixed the product in a container that was used with milk the other day so a little milk might be in the product." or "we knowingly throw a small amount of milk in." What does this, "may contain traces of milk", really mean? I'm curious because I know in some cases, like fat free products their actually allowed to add like .5 percent of fat which is really funny.