Somehow this culture has survived for decades, despite short-term thinking and too-narrow optimization destroying the staff and retail experience at other grocery stores. I'd be enormously proud to have achieved something that lasted like that. I wish I knew how they did it!
For example, I'm usually shopping at Edeka (higher tier, more SKUs, more expensive; a full-sortimenter).
Every time I shop there, there are several articles their bar code scanner does only recognize after two or three attempts, often the cashier has to try a handheld scanner, sometimes he must enter the digits by hand.
Every five or so times I shop there, an article is not recognized by the system, and the cashier must phone the department and ask for the price.
Aldi has larger bar codes on their packaging, often one on every side. Aldi barcodes are usually black on white, not black on transparent (with the content confusing the scanner). There is never an article missing in the system.
That's why the cashiers handle your articles faster than you can bag them. Because scanning works on the first try, every time.