An unexpected item

An original literary non-fiction-style account of using a self-service checkout with a small supermarket shop.

I chose the self-service checkout because I had only four things and a confident estimate of my own competence. The first three passed across the reader with cheerful little notes. Each sound seemed to congratulate me on recognising an object I had personally chosen from a shelf.

The fourth was a bag of loose oranges. I placed it where I thought it belonged. A voice informed me that an unexpected item was present. This seemed ungenerous. I had expected the oranges throughout the entire journey from the fruit aisle. I removed the bag. The voice requested assistance, having apparently concluded that my participation was making matters worse.

A shop assistant arrived. She pressed a button, moved the bag a few centimetres and asked me to try again. I wanted her to see that I had understood the first three items. I pointed to them in the bagging area. She nodded with the calmness of somebody who did not need a demonstration of my ability to buy soap.

We completed the oranges together. The machine returned to its bright and indiscriminate friendliness. The assistant left to help someone at the next screen, where another customer was explaining that he had certainly put the bread down. I paid, collected my bag and waited for the final note. It sounded exactly like the first three. The machine had not thought any more highly of me at the beginning than it did now.

How does the writer use language to create humour about competence and the checkout?