What often happens is that when people weigh things at home on their bathroom scales, they often don't get the whole item on the scales and end up taking some of the weight themselves as they try to balance the item. Also the unless the bathroom scales are on a firm hard surface, this can lead to small errors too.
I know some Airport scales are slightly out, and often if you look at the display before putting anything on them, you can see how much out they are, however I would say that in general they are more accurate than one's bathroom scales.
