This is probably skewed heavily by the credits system that Audible uses. Combined with the bundle where the Audible and Kindle versions are discounted together, it is often very low friction to get the audio version.

Of course, this is where my question of this view comes in. If you cut out Audible, how certain is Sanderson that the same number of audio books would be purchased? If it cuts over half of the sales, than that more than justifies the lower cut in profit, no?