If you see shelving in a store with signs all over it saying hats 90% off, would you assume all hats on those?
I would have, and always do, point out the discrepancy. We're not rolling in dough, we do okay, but if I see something that is supposed to be 90%, and it's almost always something I would only buy because it is 90% off, and the sign says 90% off, that's what I'm paying, or I'm not buying it. I don't argue about it, I simply ask why it is not discounted as it is marked. Sometimes it is a mistake on their computer.
I realize there are times when merchandise has been moved, and I allow for that, or the signs say "up to 90% off" but if this was clearly not the case, then give me the discount, or keep your merchandise.