From kasabe, Tales From Retail:
I work at a national retailer in the electronics department. I had just finished up helping a customer when I spy one of my managers talking to a lady at the front counter. Another one of my coworkers had helped her pick out a blu ray player and she returned after another few minutes of shopping to purchase everything at once. With nothing else to do, I stood waiting for another customer to swing by, but listened in on the conversation between this lady (L) and my manager (M).
L: "I'd like 15% off everything, please."
M: "I'm afraid I can't do that, ma'am."
I turn around. The lady has a pulled out a paper bag from a national hardware chain's store. The bag had print on it that allowed for 15% off whatever you could fit in the bag.
L: "What do you mean? (Another, different national retailer known for appliances) let me do the 15% off."
M: "Ma'am, we are not (national retailer with the bag) nor (appliance retailer)."
L: "This is ridiculous."
M: "If the item was 15% off in their ad we could price match their ad. But I can't do everything for 15% off that you have here."
The lady then grumbles and immediately takes off for the front of the store, which I could only assume was to complain to customer service about it.
Me, to M: sarcastically "Oh sure, ma'am, we can give you 15% off of every iPad that you can fit in the bag! But of course!"
TL;DR: Storewide percentages off do not carry over from one retailer to another, strangely.