I am sure there's a good reason why Spark's motors are set at an angle.  The blades push the air flow downward into a "V".  This perhaps gives it better lateral stability, and helps keep it from drifting laterally in "Sports Mode".  Anyone has any further ideas?

There's no cement explanation as to the difference between angles of the motors. You will find the best info for this against FPV racers. They can set the angle of the motors depending on how they are going to race. See the link I found for more info -http://blacktieaerial.com/the-physics-of-quadcopter-flight/