Ok, I am about to give you a little anatomy lesson that pertains to mountain biking. As you know balance is derived from your inner ear. The Semicircular Canals and Vestibule of the inner ear are filled with fluid that is responsible for balance and equilibrium. When your head is up the fluid is better equalized then when you head is down. The perilymph fluid moves forward ergo it is harder for your sensory nerves to tell the body where it is in relation to gravity and the earth.
Bottom line is head up equals better balance. Try standing on one foot with your eyes closed and look both up and down. Feel a difference?