Every card is a memory test and a design test at once: the backs have to be interesting enough to look at while you think, and the faces have to be readable at a glance.
You are not remembering the cards. You are remembering where they were, and where they were is a pattern — a row, an edge, a symmetry you noticed without noticing. This is the same mechanism behind the cocktail-party effect, behind the face you half-know, and behind the reason a shuffled deck feels different from a stacked one.
The Hint button shows you one pair, which feels like cheating and is in fact a gift: the match is easier when you have something to attach the memory to, and the pattern is what you are really learning.