...because when you step on the scale and it says that you have gained 0.2 pounds the ladies will chastise you for wearing jeans. "You don't wear jeans to weigh in!" they will exclaim. "Do you have anything on under those jeans? You could take them off." And because you are not willing to expose your flower printed Fruit of the Looms to the rest of the group you almost resign yourself to accepting the small gain.
...until your mother offers to take her lighter material shorts off in the bathroom so you can weigh yourself in those. And so, while your mother stands in the public washroom in HER flower printed Fruit of the Looms, you trot out to the scale in shorts that are three sizes too large BUT MUCH LIGHTER than your jeans and manage to lose 0.6 pounds. Victory is now your best friend.
...and jeans your mortal enemy!
(caveat: the picture above does not acurately represent my waistline in a pair of jeans. Obviously, I would not be caught dead in a pair this wrinkly. Otherwise the image is perfect)