Why do I know more than a WalMart employee?

So, today we decide to go to WalMart, cuz you know, they have things we needed and I wanted to go before Irma got here. I don't normally do things like that because we are prepared for things like this way in advance but we had to pick up my husband's medicine and were already out. It could have waited until the storm was gone and life was "back to normal" but like I said, we were already out.
I have been in a great mood all day, we are safe and happy. No problems at all. We get all our things and start to go to the checkout. Then, I decide I want some apples. I picked my apples and then we went to the register to pay for our things and the lady is ringing up all the products. She gets to the apples and swings them over the scanner, once, twice, three times. I knew they had no bar code but didn't really pay attention because I thought she was looking for the code to key it in. So, I show her where the code is. She gives me a dirty look and says I always see if there is a bar code first. I'm like ok, cool your jets lady. I didn't actually say that but things do tend to go through my mind while dealing with others. The majority of the time I don't say them out loud. So, she keys in the code with the apples barely on the scale. She say's that isn't right. I tell her you have to put them on the scale. She then gives me another dirty look and asks me if I want to do it. I started to tell her I could do a better job but keep my mouth shut. I don't understand why I had to tell her how to do her job. Aren't these people trained at all? I have never worked at WalMart but I know more about how to do things than they do. Maybe that is why people are always asking me if I work there while I'm shopping. Anyway...
As we are walking out I ask my husband if he saw what was going on and he said yes. I asked if I were wrong because my husband will tell me if I am in the wrong. He says no, she was the one who was in the wrong. She was rude and I just kept my mouth shut and didn't let her steal my great day. So, I guess the moral of this story is, don't let the crabby checkout lady steal your great day, even when a storm is coming your way.