My strong suspicion is someone misclicked a select box in a form somewhere.
Maybe. That’s clearly the likely case for Norfolk.
Do you know if tariffs are assessed based on the country of origin in the bill of lading? The Guardian article doesn’t answer that question. I’d suspect so since that’s how everyone is counting imports and exports, but the Guardian article isn’t clear.
...and nobody bothered checking?
You've never mislabeled a package or letter?
The postal/shipping/logistics service will almost certainly have gone "that's not right..." and quietly fixed it.