Originally Posted by Smoothie104
Its probably the extra weight of the valve stem and cap, since there is no counter weight across from it, accept maybe some stickers. If you pull the tube out then the other side of the rim will be slightly heavier since there is no hole in that side.
Actually, the rim opposite the valve stem is usually the heaviest point. Rim manufacturers use steel pins to pin the extrusion at the joint which is usually exactly opposite the valve hole. It's easy to tell on a front wheel just hold it off the ground and see where the wheel stops spinning. I usually put computer magnets on a spoke near the valve stem to minimize the imbalance.