The previews show a random selection of cards rated (in my experience) between 4.60 and 4.74(?). Many girls get the same rating across their cards (half of my previews seem to be Katya Clover and Melissa, even though I don't buy ther cards). It's definitely not entirely random, but I doubt it's as tricky as using some form of enhanced learning to judge what you'd like based on your last purchases.