You need to think about airspeed, groundspeed and air density. In laymans terms, (for the pedants). Both aircraft will be doing the same speed through the air. As you climb through the Earth's atmosphere the air becomes less dense, that is to say, the molecules of air are further apart the higher up you go. The aircraft at 37000' will have the same speed through the air as the one at sea level but it will need to travel further to get the same amount of air molecules to pass over it's wings to give it this same airspeed. Therefore it will have a higher speed over the ground. Make any sense?