Because private scope is limited to the class, not the instance as defined in the C# spec:
1.6.2 Accessibility
Each member of a class has an associated accessibility, which controls
the regions of program text that are able to access the member. There
are five possible forms of accessibility. These are summarized in the
following table.
Accessibility Meaning
public Access not limited
protected Access limited to this class or classes derived from this class
internal Access limited to this program
protected internal Access limited to this program or classes derived from this class
private Access limited to this class
