To IDispatch or not to IDispatch, that is the question

Recently I got strange situation in our application. We have two different classes in .NET that exposed to managed code via COM interfaces. Both classes look very similar but one class by some reason supports IDispatch while another class does not. We have piece of code that work quite differently depending on IDispatch support. And was asked to investigate what is going on and why these two classes behave different.

Here is some data. First class looks like this:

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
public class Foo : BaseClass1, IDisposable

Second class looks like this:

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.None)]
public class Bar : BaseClass2, IMyInterface

IMyInterface declared like this:

[ComVisible(true)]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("XXX")]