What a jerk! Some people just can't stand to be corrected and seen to be wrong. Even if they recognize their mistake, they refuse to correct it, instead becoming huffy or angry or self-righteous. Since THEY can't be wrong, obviously YOU must be wrong. (I'm afraid I've seen this trait more in men than in women, but that's just my experience.)
How frustrating. I guess if you had your kids there and didn't want to make a big issue of it, you didn't have much choice in how to handle this.
Maybe you should drop a friendly, educational note to the store manager about this incident. It's one thing for an employee to make a mistake; it's quite another entirely to persist in making the error when he has been politely corrected.