Crappy Design

Exactly, they rely on absentmindedness and habit..
Same deal as shrinkflation - the package and price are the same, so you assume it's the same as always, but there is less in it now than there was before, and you only notice if you pay close enough attention (which they also make sure you don't not only by using distracting graphics and design, but shit like the music in store, shelving "design" and so on, and if you go even deeper - by working you too hard to have brain power left at the end of the day to notice they're fucking you over. Perfecting ways to manipulate customers is a multi trillion dollar industry).