This can be because the actual deal itself exceeds expectations (for example, the percentage off is so unbelievable) or because the way the deal is framed makes it seem that way.

Quantity limits work the same way. Retailers sometimes create limits around the number of a given discounted item a given customer can buy. “One per household” or “Limit three per customers.” You might think that by making it harder for people to get as many as they want these restrictions would hurt demand. But they actually have the opposite effect by making the promotion seem like an even better deal.

-Contagious, p.168