If I sell lots of identical goods, it's totally clear what the law is entitling prospective customers to buy. But if I make and sell creative works, I'm naturally making something different for each client. How could anyone decide whether the work I do on one project is "equivalent" to the work done on another, or if a completed workpiece is up to my "usual standard", or even whether I put in "the same amount of effort" for this client as I did for a different client, when by definition each project is one-of-a-kind?