Cash strapped, but also presumably more likely than the general population to be innumerate or have dyscalculia or dyslexia.

It's the same bullshit that allows discount prices on Black Friday or during January sales to be completely misleading.

In the UK we are much tougher on this kind of manipulative pricing, but you still find manipulative things, like being unable to find the price-per-100g on discounted items and "clubcard" items, or bulk buys that end up having higher unit costs and yet seem not to be errors.