When GPD growth slows, the only way to net personal gain is through another's loss. If you assume well being does have some sort of financial price tag tied to it, then this makes sense that someone's well being would go down as another's goes up, as the net for the whole economy doesn't change.
The zero sum delusion assumes someone only gets rich by taking from another. The delusion is that this is the ONLY way to generate wealth. When you create new markets and new products, the economy grows and everyone is better off. When the economy doesn't grow, the only way one person is better off is someone else is less well off. These are two different cases.
