If I buy a car and the month later that one have a special discount, I don't event think to go to the seller claiming he must refund me the difference in price. Got the point?

Maybe someone underage and not drive cars, I'll try to use terms that (hopefully) can be better understood: if you buy (and eat!) your Hamburger meal at McDonald's for 2$, and the day after there is a special sell at $1,50, if you are sane on mind you don't go there asking for a $0,50 refund.