为什么在Equals方法中忽略Uri的Fragment?

我试图根据对象的URI维护一组对象:

public class ConceptCollection : KeyedCollection<Uri, Concept> {
    protected override Uri GetKeyForItem(Concept item) {
        return item.Uri;
    } 
}

ConceptCollection wines = new ConceptCollection();
Concept red = new Concept("http://www.w3.org/2002/07/owl#RedWine");
Concept white = new Concept("http://www.w3.org/2002/07/owl#WhiteWine");
wines.Add(red);
wines.Add(white); // Error: An item with the same key has already been added.

每 http://msdn.microsoft.com/en-us/library/f83xtf15.aspx :

不考虑用户的实例 http://www.contoso.com/index.htm#search 和 http://user:password@www.contoso.com/index.htm , Equals方法将返回true。

我接受了不得不破解这个问题。但它为什么会这样呢?我可以看到用户信息的逻辑,但无法看到片段的逻辑。