Why is the chocolate at the bottom of a Reese's Peanut Butter Cup often melty?

Stored at room temperature, 70 degrees F. The rest of the chocolate is firm, the crinkly edge around the top is crisp even. So why is the chocolate on the bottom of the cup noticeably softer and stick to the wax paper, to the point of being pulled off? This is a consistently reproducible phenomenon.

And no, I don’t want to refrigerate/freeze ‘em. ;-)