Track kind for audio is "description" but "descriptions" for text?

Is there a reason for the description "kind" to be named differently for
audio and text?

> "|description|" 	An audio description of a video track. 	Audio only.
> Ogg: "audio/audiodesc".
>

> |descriptions| 	Descriptions 	Textual descriptions of the video
> component of the media resource
> <http://www.w3.org/TR/2012/CR-html5-20121217/embedded-content-0.html#media-resource>,
> intended for audio synthesis when the visual component is obscured,
> unavailable, or not usable (e.g. because the user is interacting with
> the application without a screen while driving, or because the user is
> blind). Synthesized as audio.
>