Maybe they're all stories about winter, to help you keep your cool.
Or maybe they're all stories about summer, to help you get in the mood.
Or maybe they're all short, because the nights are shorter so you presumably have less time inside reading because you're going to be outside enjoying the weather.
Or maybe they're all long, so you have something to fill those long days when it's too hot to go outside.
Or maybe they come with water proof covers - to protect at the beach and from sweaty fingers.
Or maybe they didn't sell over winter, so they assume they must be books for summer.