I'm only guessing, but the air gets thinner as you go up. I would think that if the air got thinner than the weight might feel a little lighter. Also, as you get higher gravity probably is less.
On the other hand the air is thinner so you would probably have less than optimal workouts until you acclimated due to poor oxygen cinsumption coming from the thin air and your heart and lungs having to work harder to get the normal amount of oxygen going.
This is just me guessing, I don't know any physics.