In case anyone didn’t notice, the label “normal” doesn’t necessarily mean a healthy person, but a person without Alzheimer’s disease (This doesn’t rule out other neurodegenerative diseases!)
For example, as shown in my explainer notebook the percentage of “normal” samples who missed at least 1 digit is more than 50%.
Of course if you choose a healthy person at random and ask them to draw a clock you won’t find half of them missing any digits.
Hope this helps!