Donkeys have a speed attribute called "horse.movementSpeed" which is supposed to always be 0.175.  They also have "horse.jumpStrength" which is supposed to always be 0.5.  All wild donkeys are in fact 0.175 speed and 0.5 jump, but when I breed two wild donkeys together, the speed is bigger than 0.175 (often about 0.195) and the horse.jumpStrength is about 0.58.  Breeding two of those donkeys together creates an even better donkey, and so on.

This also occurs for mules; when I breed a horse and a donkey to get a mule its speed varies depending on the speed of the horse, donkey, and randomness.

Is this a bug or does it work as intended?