It's worth adding that this is absolutely consistent with other usages of "unknown" in D3D; in context "unknown" doesn't mean "I don't know it", it means "I don't specify it, the runtime or driver gets this information from elsewhere".
For example, in D3D9 when creating a windowed mode you would specify D3DFMT_UNKNOWN for the backbuffer format, meaning that it actually comes from the format of the current display mode.
It could probably be argued that use of the word "unknown" for this is confusing, but done is done. So whenever you see "unknown" it's the API design's way of telling you that this is something that you don't specify because the information comes from elsewhere and may be constrained by the state of that elsewhere.