Imagining something and materializing that something are two very different things.
The only people that can draw something perfectly from memory are savants and animators that drew the subject thousands of times as part of their jobs.
Otherwise the mind and memory are very fluid things. See; why eyewitness testimony isn’t as concrete as some people imagine.
I wonder what kind of tablets they're using? I can never get used to drawing on a USB tablet and having it appear on the screen, I can draw better with a mouse. Direct draw would be better.