Class member inheritance structure

[skataben]

I'm just beginning the comprehend the source code. I noticed what seems to be a significant amount of unnecessary code involving class hierarchy and redundant members, so I have a few questions.

The class BaseStreamCommons has a public member called "format".

The class AudioStream is derived from BaseStreamCommons and has a public member called "Format":

///<summary>The format or container of this file or stream.</summary>
[Description("The format or container of this file or stream."), Category("AllStreamsCommon")]
public string Format { get { return this.format; } }

Why give AudioStream this member at all? Why not give BaseStreamCommons the "Format" member?