That's the definition of Vx and Vy. Where Vy is defined as the greatest difference between thrust horsepower required and thrust horsepower available and Vx is greatest difference between power required and power available. Thrust Horsepower= Thrust x Velocity. Where V is equal to TAS. That being said I am not sure of the cause of the Vx Vy switch but if it's in the AFM POH then it's authoritative. Check out HHH Jr. Aerodynamics for Naval Aviators for more information.