The thing is, everybody thinks they are impervious to advertising. Everybody thinks they always choose the "cheapest option of greatest quality, based on independent research." They're just mostly wrong, is all. They either don't understand the way their purchasing decisions actually get made, or they choose to tell themselves they are made differently than they are.

Which isn't to say that you're not the exception, just that generally speaking everybody thinks they're the exception. So how you think you buy things doesn't mean much; you'd have to have someone outside your own head look at your purchasing patterns to know for sure.