Cool pillowcases

Even if it's winter and the house is cold and I'm piled under a number of blankets, I still find myself turning over the pillow to try to find a cool spot. The pillowcase I have now isn't as good at having cool spots as other ones I've had (but it has other virtues).

I've heard other people do the same thing, but what I don't know is why. What is it about pillowcases that makes it nice to have a cool spot even when you're cold? What is the appeal?