That’s not to say everyone in the UK knows every first part of the post code off by heart mind. Same way that most US residents likely couldn’t tell you where a random zip code was without looking it up - you get used to the ones in your local area but not generally any outside that unless you spend some time elsewhere enough to warrant it.

My wife and I see this all the time. We’ve made a game of it. “Choices were made” - can get interesting imagining why someone’s dropped the other item there (like a food item in with the cleaning supplies…)

I just have to add 20 on to get mine, so not too bad.