The monster that is consumerism

Why is it that when a normal person enters a shop and assumes the role of "consumer" they instantly turn into an asshole?

I was working the other day, the shop I work at is well known for its affordable price. My last-hour was Ladies Fashion. This is the conversation that happened:

Register Three: Attention staff price check Ladies Fashion to register three please.
Me: *I walk up to the counter*.
Register Three: Hey can you see if these pants are 26.99?
Me: Sure, I'll be back in a moment.

Then I walk back to Ladies Fashion, the pants that I were shown were these jeans that had a glittery belt attached. I found them along the wall, where almost all of the jeans are at the moment. Above them is a ticket frame that says the pants were originally 39.99 but now they are 29.99.

I walk further along the wall and then I discover two pairs of the pants I was looking for dumped on a rack that was for these other jeans that have a tan colored belt. The ticket frame on this rack said the pants were originally 35.99 but now they were 26.99. So I move the glittery belt pants to their right place and walk back to the registers.

Me: No sorry, they're 29.99.
Customer: No they're 26.99.
Me: No sorry, the sign says 29.99.
Customer: I don't think so! It said 26.99.
Me: No...
Customer: Don't worry I'll just buy them anyway.

So I went back to Ladies Fashion to continue cleaning. The customer then walked down to where the jeans.

Customer: They've changed the signs around! This sign was there before. This one had different writing, it was smaller.

The printer only has one sized font.

Her daughter tried to make her talk quieter because it was obvious I could hear.

Customer: Excuse me! You've changed the signs around, this one was there.
Me: No I didn't.
Customer: You did! Because these are the pants *points to the pants I moved* my daughter tried on! We put them back here after we went to the fitting rooms. These are the exact pants. You have to give it to me for 26.99.
Me: No I don't. That's not the price for those pants. These pants were not orignally 35.99, they were 39.99. They must have been on the wrong rack.
Customer: You have to give it to me for the right price.
Me: Fine I'll just go get my manager *rolls eyes*.

As it turns out my manager was on the phone. He said he'd be up in a minute. So I walked back to the customer.

Me: Sorry, he's on the phone at the moment, he said he'd be up here shortly.
Customer: LOOK! DON'T WORRY ABOUT IT! IT'S JUST THE PRINC....

I walked off. I really can't stand being yelled at, ever. Especially when I'm right. Like these customers just cannot be told. They're right, you're wrong and there's nothing you can do about it. But seriously, how would lying to her benefit me? There's no need for me to lie or rip anyone off.

I don't even do it unintentionally and infact I've only ever ripped someone off unintentionally once, and then they wouldn't let me fix it by giving them a refund.