I think sometimes our preconceptions affect how we "see" things. If someone tells me , say, Walmarts fabrics are no good" (not that I think that, I buy there all the time) then the next time I go there, I'm going to be looking at it with a critical eye and if I have any doubts they're going to be pushed to the negative side.
I'd loooove to see a totally blind test to see if people really can tell a difference, that would be very telling, I think.