I've googled quite a bit, and found it weird that no one cared to explain why COM interfaces are immutable. I suppose that the reason you're unable to remove any methods from a COM interface, is because a client relying on that interface, would encounter an error, which isn't good. But why would adding new features to the interface change any of that? Has this something to do with the underlying vtable?
If you add a method, a newer client that uses that method will fail when working with an older version of the component.
Older versions of the component won't have the new method unless you specifically add code to implement it, rebuild the component, and then reinstall the component on all machines that use the component.
If a newer version of the client attempts to call the new method on an older version of the component that doesn't have the method, undefined behavior will occur (likely a crash but silent data corruption is also possible). The new client will be attempting to call a method through a pointer entry in the vtable that did not exist when the old client was built so the old client will have some unrelated value in this location.
Of course, this wouldn't be an issue for you if you control both client and component and deploy them together but COM is designed for a much broader range of use cases than that.