You'd think that, but there are some extra forces at work.
There's a certain amount of friction at the point where the strings come into contact with the slots in the nut. When you tune a string up, you're pulling hard enough from the headstock side that the part of the string south of the nut (fretboard side) is taut enough to get the proper tone. As time goes by, the extra pull from the headstock side overcomes the nut friction and the string gets pulled a bit tighter, raising its pitch.
--Mark