I'm not completely sure, but I think the idea might be that if you've come, got a price, left, come back later to get a price for the same thing, then between leaving and coming back you've shopped around and settled on them. Since you've settled on them your intent now is to buy from them, so they try to get more money. The thing that confuses me though is that if you're shopping around you're probably price sensitive enough to notice?