The science is about how you initially react to the number. Your brain will see $19, and immediately you'll think it's $19. Only upon further inspection and processing through your cognition, you recognise that its $19.99, which is basically $20.

It's that initial reaction they want, to grab your attention. Anyone who is going through life without leveraging their higher thinking will fall for this shit. Anyone who thinks, at all, won't.

Unfortunately, there's a nontrivial number of people who fall into that first category. People who were never taught to think. They just do.